HYBRIDON INC
S-1/A, 1999-07-07
BIOLOGICAL PRODUCTS, (NO DIAGNOSTIC SUBSTANCES)
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As filed with the Securities and Exchange Commission on July 7, 1999
                                                      Registration No. 333-69649
- --------------------------------------------------------------------------------

- --------------------------------------------------------------------------------


                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                            ------------------------

                                 AMENDMENT NO. 1
                                       TO
                                    FORM S-1
                             REGISTRATION STATEMENT
                                      UNDER
                           THE SECURITIES ACT OF 1933

                            ------------------------


                                 HYBRIDON, INC.
             (Exact Name of Registrant as Specified in Its Charter)
<TABLE>
<CAPTION>

                     Delaware                                    2836                                 04-3072298
<S>              <C>                                      <C>                                      <C>
          (State or Other Jurisdiction of            (Primary Standard Industrial                  (I.R.S. Employer
          Incorporation or Organization)              Classification Code Number)               Identification Number)
</TABLE>

                                155 Fortune Blvd.
                          Milford, Massachusetts 01757
                                 (508) 482-7500
               (Address, Including Zip Code, and Telephone Number,
        Including Area Code, of Registrant's Principal Executive Offices)

                            ------------------------


                            E. ANDREWS GRINSTEAD, III
          Chairman of the Board, President and Chief Executive Officer
                                 HYBRIDON, INC.
                                155 Fortune Blvd.
                          Milford, Massachusetts 01757
                                 (508) 482-7500
            (Name, Address Including Zip Code, and Telephone Number,
                   Including Area Code, of Agent For Service)

                            ------------------------


                                    Copy to:

                              MONICA C. LORD, ESQ.
                       Kramer Levin Naftalis & Frankel LLP
                                919 Third Avenue
                            New York, New York 10022
                                 (212) 715-9100


                            ------------------------

                        Approximate date of commencement
                      of proposed sale to the public: From
                      time to time after this registration
                          statement becomes effective.

            If any of the  securities  being  registered  on this Form are to be
offered  on a  delayed  or  continuous  basis  pursuant  to Rule 415  under  the
Securities Act of 1933, check the following box. |X|

            If this  Form is  filed to  register  additional  securities  for an
offering  pursuant to Rule 462(b)  under the  Securities  Act,  please check the
following box and list the Securities Act  registration  statement number of the
earlier effective registration statement for the same offering.
|-|

            If this Form is a  post-effective  amendment  filed pursuant to Rule
462(c) under the Securities Act, check the following box and list the Securities
Act  registration   statement  number  of  the  earlier  effective  registration
statement for the same offering. |_|

            If delivery  of the  prospectus  is expected to be made  pursuant to
Rule 434, please check the following box. |_|




<PAGE>

<TABLE>
<CAPTION>

                                                   CALCULATION OF REGISTRATION FEE
===================================================================================================================================
                                                    Proposed           Proposed
                                                    Maximum            Maximum
 Title of Each Class         Amount to              Offering           Aggregate        Amount of          Amount
 of Securities to be             be                 Price Per          Offering        Registration      Previously       Amount
      Registered             Registered               Share             Price              Fee              Paid            Due
- -----------------------------------------------------------------------------------------------------------------------------------
<S>                           <C>                 <C>                <C>               <C>               <C>             <C>
Series A Convertible
Preferred Stock,               662,099               100 (2)         $66,209,900       $20,063.59        $19,430.15      $633.44
$.01 par value
- -----------------------------------------------------------------------------------------------------------------------------------
Common Stock,               10,195,175           1.15625 (3)          11,788,171        $3,571.82         $3,571.82           $0
$.001 par value
- -----------------------------------------------------------------------------------------------------------------------------------
Common Stock,                  460,000           0.54687 (4)             251,560           $76.23              $0         $76.23
$.001 par value
- -----------------------------------------------------------------------------------------------------------------------------------
Common Stock,
$.001 par value,
issuable upon                  173,333              3.00 (1)(6)          519,999          $157.58              $0        $157.58
exercise of Forum
Warrants
- -----------------------------------------------------------------------------------------------------------------------------------
Common Stock,
$.001 par value,
issuable upon               15,578,800
conversion of Series            (1)                   -- (5)                  --                --                --          --
A Convertible
Preferred Stock
- -----------------------------------------------------------------------------------------------------------------------------------
Common Stock,
$.001 par value,
issuable upon                3,002,958              4.25 (1)(6)       12,762,571        $3,867.05         $3,867.05           $0
exercise of Class A             (1)
Warrants
- -----------------------------------------------------------------------------------------------------------------------------------
Common Stock,
$.001 par value,
issuable upon                1,752,945              2.40 (1)(6)        4,207,068        $1,274.74         $1,274.74           $0
exercise at Class B             (1)
Warrants
- -----------------------------------------------------------------------------------------------------------------------------------
Common Stock,
$.001 par value,
issuable upon                  904,274 (1)          2.40 (1)(6)        2,170,257         $657.88           $657.88            $0
exercise of Class C
Warrants
- -----------------------------------------------------------------------------------------------------------------------------------
Common Stock,
$.001 par value,
issuable upon                  672,267 (1)          2.40 (1)(6)        1,613,441         $488.87           $488.87            $0
exercise of Class D
Warrants
- -----------------------------------------------------------------------------------------------------------------------------------
Common Stock,
$.0001 par
value, issuable upon           609,195              2.40 (1)(6)        1,462,065         $443.01           $443.01            $0
exercise
of Forum Warrants
- -----------------------------------------------------------------------------------------------------------------------------------
Common Stock,
$.0001 par value,
issuable upon                  588,235              4.25 (1)(6)        2,499,999         $757.50           $757.50            $0
exercise of Forum
Warrants
- -----------------------------------------------------------------------------------------------------------------------------------
Common Stock,
$.0001 par value,
issuable upon                1,111,630              2.40 (1)(6)        2,667,912         $808.38           $808.38            $0
exercise of Pillar
Investment Warrants
===================================================================================================================================
</TABLE>

                                        2



<PAGE>

(1)         Pursuant  to Rule 416  Hybridon is also  registering  that number of
            additional  shares of Common Stock that may become issuable pursuant
            to applicable anti-dilution provisions.

(2)         Estimated  solely for purposes of calculating the  registration  fee
            using  the  proposed  offering  price of the  Series  A  Convertible
            Preferred  Stock,  as required by Rule 457(i).  Does not include any
            shares of Series A  Preferred  that may be issued in the future as a
            dividend, which shares are expressly excluded from this Registration
            Statement pursuant to Rule 416(b) under the Securities Act.

(3)         Estimated  solely for purposes of calculating the  registration  fee
            using the  average of the bid and ask price for the Common  Stock on
            December 17, 1998 as required by Rule 457(c).

(4)         Estimated  solely for purposes of calculating the  registration  fee
            using the  average of the bid and ask price for the Common  Stock on
            June 29, 1999 as required by Rule 457(i).

(5)         Pursuant to Rule 457(i) no additional registration fee is required.

(6)         Estimated  solely for purposes of calculating the  Registration  Fee
            using  the  exercise  price of the  Warrants,  as  required  by Rule
            457(g)(1).

                         ------------------------------

            The  Registrant  hereby amends this  Registration  Statement on such
date or  dates as may be  necessary  to  delay  its  effective  date  until  the
Registrant shall file a further  amendment which  specifically  states that this
Registration  Statement  shall  thereafter  become  effective in accordance with
Section 8(a) of the Securities Act of 1933 or until the  Registration  Statement
shall become  effective on such date as the Commission,  acting pursuant to said
Section 8(a), may determine.



                                        3



<PAGE>

                   SUBJECT TO COMPLETION, DATED JULY [6], 1999


The  information in this  prospectus is not complete and may be changed.  We may
not sell  these  securities  until the  registration  statement  filed  with the
Securities and Exchange Commission is effective. This prospectus is not an offer
to sell these  securities and is not soliciting an offer to buy these securities
in any state where the offer or sale is not permitted.

                                 HYBRIDON, INC.
                             155 Fortune Boulevard
                          Milford, Massachusetts 01757

                         Secondary Offering Prospectus

             662,099 shares of Series A Convertible Preferred Stock

                                      and

                       35,048,809 shares of Common Stock

            This  Prospectus  offers for  resale by the  holders  (the  "Selling
Security  Holders") in transactions  registered under the Securities Act of 1933
662,099  shares  of Series A  Convertible  Preferred  Stock,  $.01 par value per
share, of Hybridon (the "Convertible Preferred Stock"), and 35,048,809 shares of
the Common Stock,  $.001 par value per share, of Hybridon (the "Common  Stock").
These shares were originally  issued in transactions  intended to qualify for an
exemption from registration under the Securities Act.

            In this Prospectus,  the Convertible  Preferred Stock and the Common
Stock are collectively referred to as the "Securities."

            Certain  of the  shares of Common  Stock  being  registered  will be
issued upon the exercise of certain  warrants issued by Hybridon.  Hybridon will
not receive any of the offering  proceeds  other than  proceeds upon exercise of
these warrants.

            The Common Stock is quoted on the NASD  Over-the-Counter,  or "OTC,"
Bulletin Board under the symbol "HYBN." Prior to this offering there has been no
public market for the Convertible Preferred Stock.

            See  "Risk  Factors"  beginning  page  5 of  this  Prospectus  for a
discussion  of  certain  factors  that you  should  consider  in  evaluating  an
investment in the Securities.

            Neither  the  Securities  and  Exchange  Commission  nor  any  state
securities  commission  has  approved  or  disapproved  of these  securities  or
determined if this Prospectus is truthful or complete. Any representation to the
contrary is a criminal offense.

                The date of this Prospectus is __________, 1999.



<PAGE>

                              AVAILABLE INFORMATION

            Hybridon is subject to the information reporting requirements of the
Securities  Exchange  Act of 1934,  as  amended  (the  "Exchange  Act"),  and in
accordance with the Exchange Act files reports, proxy and information statements
and  other  information  with  the  Securities  and  Exchange   Commission  (the
"Commission").   You  may  inspect  and  copy  reports,  proxy  and  information
statements  and other  information  at the public  reference  facilities  of the
Commission at 450 Fifth Street, N.W., Washington, D.C. 20549, or at its regional
offices  located at  Citicorp  Center,  500 West  Madison  Street,  Suite  1400,
Chicago, Illinois 60661 and 7 World Trade Center, Suite 1300, New York, New York
10048.  You may obtain copies of this  material  from the  Commission by mail at
prescribed  rates.  Please  direct  your  requests  to the  Commission's  Public
Reference  Section,   Room  1024,  Judiciary  Plaza,  450  Fifth  Street,  N.W.,
Washington, D.C. 20549. The Commission maintains a Web site (http://www.sec.gov)
that  contains  certain of  Hybridon's  reports  that were filed after  Hybridon
became an electronic filer.

            Hybridon has filed with the Commission a  Registration  Statement on
Form S-1 under the  Securities  Act of 1933, as amended (the  "Securities  Act")
with respect to the Securities  being offered by this Prospectus  (including all
exhibits and amendments hereto, the "Registration  Statement").  This Prospectus
does not contain all the information set forth in the Registration Statement and
its  exhibits  and  schedules.  Statements  made  in this  Prospectus  as to the
contents of any contract,  agreement or other document referred to summarize the
material provisions of those documents,  but are not necessarily complete;  with
respect to each such  contract,  agreement or other document filed as an exhibit
to the Registration  Statement,  please refer to the exhibit for a more complete
description of the matter  involved.  You may inspect copies of the Registration
Statement and its exhibits,  without charge, at the offices of the Commission at
the addresses set forth above.

                                        2



<PAGE>

                               PROSPECTUS SUMMARY


            This  summary   highlights   information  found  elsewhere  in  this
prospectus.  It may not contain all of the information that is important to you.
To  understand  this  offering  fully,  you should  read the  entire  prospectus
carefully,  including  the  "Risk  Factors"  section  and  Hybridon's  financial
statements.

                                    HYBRIDON

            Hybridon,  established  in 1989,  is a leader in the  discovery  and
development  of genetic drugs based on  "antisense"  technology,  in other words
drugs that use synthetic RNA and DNA, also called  "oligonucleotides,"  with the
aim of stopping or reducing the body's  production  of proteins that directly or
indirectly cause a given disease.

            Hybridon  believes that drugs based on antisense  technology  may be
more  effective,  may cause fewer side effects,  and may have a greater range of
applications than conventional drugs.

            Hybridon's leadership in this field is based on a number of factors:
its access to proprietary manufacturing technology for the chemical modification
of  oligonucleotides;  its expertise in the efficient  design and development of
antisense   drugs;   innovations   it  has   devised  in  the   manufacture   of
oligonucleotides;  its large-scale  oligonucleotide  manufacturing facility; and
its  expertise in the  manufacture  of  oligonucleotides  with diverse  chemical
modifications.

            Hybridon's drug development and discovery  programs  currently focus
on the treatment of cancer,  viral  infections  (including AIDS and AIDS-related
diseases),  and diseases of the eye.  Hybridon is currently  developing  several
drug products, including the cancer drug GEM(R) 231. Hybridon recently completed
a Phase I clinical  trial of GEM(R) 231, and the FDA recently  granted  Hybridon
approval to start a Phase II clinical  trial of GEM(R) 231.  Other Hybridon drug
products  include the AIDS drug GEM(R) 92, for which  Hybridon  has  completed a
Pilot  Phase I  clinical  study in  Europe  in which  the drug was  administered
orally.

            Hybridon is engaged in corporate collaborations, including with G.D.
Searle & Co., and also has collaborative research relationships with independent
researchers  and  academic  and research  institutions,  including  the National
Institutes of Health.

            Through  its  Hybridon   Specialty   Products   division,   Hybridon
manufactures oligonucleotide compounds both for Hybridon's internal use, for use
by collaborators,  and for sale to third parties.  Hybridon believes that demand
for  oligonucleotide  compounds will  increase,  and its strategy is to position
Hybridon Specialty Products to take advantage of this increased demand.



                                        3



<PAGE>

                             SUMMARY FINANCIAL DATA

<TABLE>
<CAPTION>

                                                                                                           Three Months
                                                                        Years Ended                           Ended
                                                                        December 31,                        March 31,
                                                     ------------------------------------------------------------------------------
                                                              1996           1997       1998           1998            1999
                                                     ------------------------------------------------------------------------------
                                                          (In thousands, except per share data)             (unaudited)
Statement of Operations Data:
Revenues
<S>                                                          <C>             <C>       <C>              <C>            <C>
    Research and development.........................        $1,419          $945      $1,100           $150           $150
    Product and service revenue......................         1,080         1,877       3,254            825          1,530
    Royalty income...................................            62            48           -              -             40
    Interest income..................................         1,447         1,079         148             18             53
                                                     -----------------------------------------------------------------------------
                                                              4,008         3,949       4,502            993          1,773
Operating Expenses
    Research and development.........................        39,390        46,828      20,977          6,403          3,447
    General and administrative.......................        11,347        11,026       6,573          1,665          1,122
    Interest.........................................           124         4,536       2,932          1,607            170
    Restructuring....................................            --        11,020          --             --             --
                                                     -----------------------------------------------------------------------------

       Total operating expenses......................        50,861        73,410      30,482          9,675          4,739
                                                     -----------------------------------------------------------------------------
Loss from operations.................................       (46,853)      (69,461)    (25,980)        (8,682)        (2,966)
Extraordinary item:
    Gain on conversion of 9% convertible
       subordinated notes payable....................            --            --       8,877             --             --
                                                     -----------------------------------------------------------------------------
Net loss.............................................       (46,853)      (69,461)    (17,103)        (8,682)        (2,966)

Accretion of preferred stock dividend................            --            --       2,689             --          1,042
                                                     -----------------------------------------------------------------------------
Net loss to common stockholders......................      $(46,853)     $(69,461)   $(19,792)       $(8,682)       $(4,008)
                                                     =============================================================================
Basic and diluted net loss per common share from:
    Operations.......................................       $(10.24)      $(13.76)     $(2.19)        $(1.72)        $(0.19)
    Extraordinary gain...............................            --            --        0.75             --             --
                                                     -----------------------------------------------------------------------------
       Net loss per share............................        (10.24)       (13.76)      (1.44)         (1.72)         (0.19)
       Accretion of preferred stock dividends........            --            --       (0.23)            --          (0.07)
                                                     -----------------------------------------------------------------------------
       Net loss per share applicable to common
          stockholders...............................       $(10.24)      $(13.76)     $(1.67)        $(1.72)        $(0.26)
                                                     =============================================================================
Shares used in computing basic and
    diluted net loss per Common Share (1)............         4,576         5,050      11,859          5,060         15,305

</TABLE>

<TABLE>
<CAPTION>

Balance Sheet Data:                                              December 31,                            March 31,
                                                     ------------------------------------
                                                              1997          1998                          1999
                                                     ------------------------------------            -----------------------
Cash, cash equivalents and short-term
<S>            <C>                                           <C>           <C>                           <C>
    investments(2)...................................        $2,202        $5,607                        $2,461
Working capital deficit..............................       (24,100)       (5,614)                       (7,820)
Total assets.........................................        35,072        16,536                        12,769
Long-term debt and capital lease
    obligations, net of current portion..............         3,282           473                           454
9% convertible subordinated
    notes payable....................................        50,000         1,306                         1,306

Accumulated deficit..................................      (218,655)     (238,448)                     (242,456)
Total stockholders' equity (deficit).................       (46,048)        2,249                          (656)
</TABLE>

(1) Computed  on the  basis  described  in  Notes  2(b)  and  19(c)  of Notes to
    Consolidated Financial Statements appearing elsewhere in this Prospectus.

(2) Short-term   investments   consisted  of  U.S.  government  securities  with
    maturities  greater  than  three  months  but less  than  one year  from the
    purchase date.

                                        4



<PAGE>

                                  RISK FACTORS

            Investing in the Securities  involves  substantial risks. You should
consider  carefully the following  risk factors,  together with all of the other
information  included in this  Prospectus,  in deciding whether to invest in the
Securities.

Hybridon May Never Generate Revenues from Sales of Its Drugs

            Hybridon's  business  is at  an  early  stage  of  development,  and
Hybridon has not yet  generated  any revenues  from the  commercial  sale of its
drugs. Due to the various risks inherent in Hybridon's business and described in
the following risk factors,  Hybridon may never  generate  revenues from sale of
its drugs, and may never become  profitable.  See  "Management's  Discussion and
Analysis  of  Financial   Condition  and  Results  of   Operations--Results   of
Operations" and "-- Liquidity and Capital Resources."

Hybridon Has a History of Operating Losses and Anticipates Future Losses

            Hybridon has never earned a profit and has incurred  substantial net
operating  losses.  These losses were caused by lack of revenues from drug sales
to offset research and development and administrative costs. Hybridon expects to
incur operating losses for at least the next several years, as it plans to spend
substantial amounts on research and development,  including  preclinical studies
and clinical trials, and, if it obtains necessary regulatory approvals, on sales
and marketing  efforts.  See "Management's  Discussion and Analysis of Financial
Condition and Results of  Operations--Results  of Operations"  and "-- Liquidity
and Capital Resources."

Hybridon May Determine  that One or More Drugs in Development  Are  Commercially
Impractical and Cannot Be Sold Commercially

            Before a drug is sold commercially,  it must go through an expensive
and  time-consuming  testing process.  Hybridon's drugs are at various stages in
this process,  and Hybridon may at any stage determine that one or more of these
drugs  cannot  be  successfully   developed.   A  drug  may,  for  instance,  be
ineffective,  have undesirable side effects,  or demonstrate  other  therapeutic
characteristics  that  prevent  or limit its  commercial  use,  or may prove too
costly to produce in commercial  quantities.  If Hybridon determines that a drug
cannot  be  successfully  developed,  Hybridon  would  not be able  to  generate
revenues from sale of that drug.

Seeking Regulatory Approval of Drugs Is Time-Consuming and Expensive; Failure to
Obtain Approval of a Drug Would Prevent Hybridon from Selling that Drug; Failure
to Comply  With  Ongoing  Regulatory  Requirements  Could  Cause  Hybridon to Be
Subject to Penalties

            Hybridon is subject to extensive regulation by numerous governmental
authorities in the U.S. and abroad.  Obtaining regulatory approval of a drug can
take several  years -- exactly how long depends upon the type,  complexity,  and
novelty of the drug -- and is typically very  expensive.  The  regulations  that
Hybridon  must comply with may change,  and may even become more  burdensome  to
Hybridon.

            Even if Hybridon is satisfied that a drug is safe and effective, the
regulatory  authorities  may not agree,  as data from  preclinical  studies  and
clinical  trials can generally be interpreted in different  ways.  Hybridon will
need the approval of  regulatory  agencies in order to sell a drug.  If they are
unwilling to grant that approval, Hybridon will not be able to generate revenues
from sale of that drug.

            Approval  of a drug  does  not end  the  involvement  of  regulatory
authorities. Hybridon and its approved drugs will be subject to continued review
and  periodic  inspection.  Approval  of a  Hybridon  drug  may  be  subject  to
restrictions  that limit how Hybridon may market that drug.  Restrictions may be
imposed on the price at which Hybridon may sell its drugs.  If Hybridon fails to
comply  with  any  regulations,  it may  be  subject  to  fines,  suspension  of
regulatory approvals, drug recalls, and other penalties.



                                        5



<PAGE>

Delays in Patient  Enrollment  Could Increase the Cost or Duration of Hybridon's
Clinical Studies

            Clinical  trials are very  costly and  time-consuming.  How  quickly
Hybridon is able to complete a clinical  study  depends  upon  several  factors,
including the size of the patient population, how easily patients can get to the
site of the clinical study, and the criteria for determining  which patients are
eligible to join the study.  Delays in patient enrollment could delay completion
of a clinical study and increase its costs,  and could also delay the commercial
sale of the drug that is the subject of the clinical trial.

Hybridon  Must Secure  Additional  Funding to Avoid  Terminating  Operations  or
Filing  For  Bankruptcy;  It May  Not Be Able to  Secure  Sufficient  Additional
Financing

            Hybridon has very limited cash resources and substantial obligations
to lenders,  its real estate landlords,  trade creditors and others.  Hybridon's
ability to continue  operations  in 1999 depends on its success in obtaining new
funds.  If Hybridon is unable to obtain  substantial  additional  new funding in
July 1999,  it will be forced to terminate  its  operations or seek relief under
applicable  bankruptcy  laws.  See  "Management's  Discussion  and  Analysis  of
Financial Condition and Results of Operations--General" and "-- Cash Resources."

            In  their  report  on   Hybridon's   December  31,  1998   financial
statements,  Arthur Andersen LLP,  Hybridon's  independent  public  accountants,
states that there is substantial doubt about Hybridon's ability to continue as a
going concern.

            Hybridon  anticipates  that,  even if it obtains  sufficient cash to
fund its operations in 1999, it will be required to raise substantial additional
funds through external sources,  including through  collaborative  relationships
and public or private financings,  to support Hybridon's operations beyond 1999.
If  adequate  funds are not  available,  Hybridon  may be forced to (1)  further
curtail significantly one or more of its research,  drug recovery or development
programs,  (2) obtain funds through arrangements with collaborative  partners or
others  that may  require  Hybridon  to  relinquish  rights  to  certain  of its
technologies,  drug candidates or drugs, (3) terminate  operations,  or (4) seek
relief under applicable bankruptcy laws.

Additional Financing May Cause Stockholder Dilution

            If Hybridon raises  additional  funds by issuing equity  securities,
the ownership  interest of existing  stockholders will be diluted.  In addition,
Hybridon  may  grant  future  investors  rights  superior  to those of  existing
stockholders.

If Hybridon Defaults Under its Loan, It Could Be Forced To Terminate  Operations
or File for Bankruptcy

            Hybridon  is  a  party  to  a  substantial  loan.  The  lenders  may
accelerate  the repayment  date of the loan in the event of default by Hybridon.
If Hybridon does default on the loan,  and the lenders  accelerate the repayment
date,  the lenders could  foreclose on Hybridon's  assets,  and this could force
Hybridon to  terminate  operations  or seek relief under  applicable  bankruptcy
laws.  Hybridon  cannot  guarantee  that it will not  default  on the loan.  See
"Management's  Discussion  and  Analysis of Financial  Condition  and Results of
Operations--1998 Financing Activities."

The "Penny Stock" Rules Will Likely Have an Adverse Effect on Your Liquidity and
Hybridon's Ability to Raise Additional Capital

            Since  the  Common  Stock is not  listed  on a  national  securities
exchange  or on a qualified  automated  quotation  system,  it is subject to the
"penny  stock"  provisions  of Rule 15g-9 under the  Securities  Exchange Act of
1934,  as amended,  which  impose  additional  sales  practice  requirements  on
broker-dealers  that sell such securities.  Prior to any transaction  covered by
this rule, the  broker-dealer  must receive from the purchaser a written consent
to the  transaction,  and must reasonably  determine that  transactions in penny
stocks are  suitable  for the  purchaser,  and that the  purchaser is capable of
evaluating the risks of transactions in penny stocks.  These  requirements  will
likely have an adverse effect on the market liquidity of Hybridon's  securities,
and

                                        6



<PAGE>

therefore on Hybridon's ability to raise funds, the ability of broker-dealers to
sell Hybridon's  securities,  and the ability of purchasers to sell any of their
Hybridon securities in the secondary market.

Hybridon May Be Unable to Obtain or Enforce Patents; Its Patents May Not Provide
Adequate Protection

            Hybridon's  success  will depend to a large extent on its ability to
(1) obtain U.S. and foreign patent protection for drug candidates and processes,
(2) preserve trade secrets and (3) operate  without  infringing the  proprietary
rights of third  parties.  Legal  standards  relating to the validity of patents
covering pharmaceutical and biotechnological  inventions and the scope of claims
made under such patents are still developing. As a result, Hybridon's ability to
obtain and enforce  patents  that  protect its drugs is  uncertain  and involves
complex legal and factual questions.

            That Hybridon owns or licenses pending or future patent applications
does not mean  that  patents  based on those  applications  will  ultimately  be
issued.  To obtain a patent on an invention,  one must be the first to invent it
or the first to file a patent  application for it. Hybridon cannot be completely
sure that the  inventors  of  subject  matter  covered  by  patents  and  patent
applications  that it owns or licenses were the first to invent, or the first to
file patent applications for, those inventions.  Furthermore  existing or future
patents  may  be  challenged,   infringed   upon,   invalidated,   found  to  be
unenforceable,  or  circumvented by others.  Hybridon's  rights under any issued
patents  may not  provide  sufficient  protection  against  competing  drugs  or
otherwise    cover    commercially    valuable    drugs   or   processes.    See
"Business--Patents, Trade Secrets, and Licenses."

Hybridon  Could  Become  Involved  in   Time-Consuming   and  Expensive   Patent
Litigation; Adverse Decisions in Patent Litigation Could Cause Hybridon to Incur
Additional Costs and Experience Delays in Bringing New Drugs to Market

            The   pharmaceutical   and   biotechnology   industries   have  been
characterized by time-consuming  and extremely  expensive  litigation  regarding
patents and other  intellectual  property  rights.  Hybridon  may be required to
commence,  or may be made a party  to,  litigation  relating  to the  scope  and
validity of its  intellectual  property  rights,  or the  intellectual  property
rights of others.  Such litigation could result in adverse  decisions  regarding
the patentability of Hybridon's  inventions and products, or the enforceability,
validity,  or scope of protection  offered by its patents.  Such decisions could
make Hybridon liable for substantial  money damages,  or could bar Hybridon from
the manufacture, use, or sale of certain products, resulting in additional costs
and  delays  in  bringing  drugs to  market.  Hybridon  may not have  sufficient
resources to bring any such  proceedings to a successful  conclusion.  It may be
that entry into a licensing  arrangement  would allow Hybridon to avoid any such
proceedings.  There can be no assurance, however, that Hybridon would be able to
enter into any such licensing arrangement on terms acceptable to Hybridon, or at
all.

            Hybridon  also  may  be  required  to  participate  in  interference
proceedings   declared  by  the  U.S.   Patent  and  Trademark   Office  and  in
International Trade Commission  proceedings aimed at preventing the importing of
drugs that would compete unfairly with Hybridon drugs.  Such  proceedings  could
cause Hybridon to incur considerable costs.

Hybridon's Trade Secrets and Other Unpatented Proprietary Information May Become
Available to Others

            Trade secrets and other unpatented proprietary  information plays an
important  role  in  Hybridon's   business.   Hybridon  seeks  to  protect  this
information,   in  part  by  means  of   confidentiality   agreements  with  its
collaborators,  employees,  and  consultants.  If any  of  these  agreements  is
breached,  Hybridon may be without  adequate  remedies.  Also,  Hybridon's trade
secrets may become  known or be  independently  developed by  competitors.  This
could have a material  adverse effect on Hybridon's  business,  and Hybridon may
need  to  engage  in  costly  and  time-consuming   litigation  to  protect  its
proprietary rights.

The Loss of Key Members of Management Could Be Damaging

Hybridon  depends on the  principal  members of its  management  and  scientific
staff,  including E. Andrews Grinstead,  III,  Hybridon's Chairman of the Board,
President and its Chief Executive Officer, and

                                        7



<PAGE>

Sudhir  Agrawal,  Hybridon's  Senior Vice  President of Discovery  and its Chief
Scientific  Officer.  The loss of their services  could have a material  adverse
effect on Hybridon.

Hybridon  May Not Be Able to Meet Its  Personnel  Needs;  This  Could  Result in
Delays or Additional Costs

            From June 30,  1997,  to June 15,  1999,  the number of employees of
Hybridon  decreased from 213 to 50. As a result,  Hybridon has lost  significant
expertise,  and must recruit and retain new scientific personnel to maintain its
current level of operations, while expansion would require a further increase in
scientific personnel. In addition,  expansion by Hybridon would likely result in
the  need  for  additional  management  personnel.  Hybridon  may not be able to
attract and retain  personnel on acceptable  terms,  given the  competition  for
experienced   scientists   and   management   among   numerous   pharmaceutical,
biotechnology and health care companies,  universities,  and non-profit research
institutions. The failure to recruit and retain personnel could result in delays
in commercializing drugs, and could cause Hybridon to incur additional costs.

Hybridon  Relies on  Relationships  With  Research  Institutions  and  Corporate
Partners,  and  Would  Be  Harmed  By  a  Lack  of,  or  the  Failure  of,  Such
Relationships

            Hybridon's  success will depend in part on its continued  ability to
develop and maintain  relationships  with  independent  researchers  and leading
academic and research  institutions.  The competition for such  relationships is
intense, and Hybridon can give no assurances that it will be able to develop and
maintain such  relationships  on acceptable  terms.  Hybridon has entered into a
number of collaborative  relationships  relating to specific disease targets and
other research activities in order to augment its internal research capabilities
and to obtain access to specialized  knowledge or expertise.  The loss of any of
these  collaborative  relationships  could  have a  material  adverse  effect on
Hybridon's research and development program.

            Similarly,  strategic alliances with corporate  partners,  primarily
pharmaceutical  and  biotechnology  companies,  may help  Hybridon  develop  and
commercialize  drugs.  Various  problems  can arise in  strategic  alliances.  A
partner  responsible  for conducting  clinical  trials and obtaining  regulatory
approval may fail to develop a  marketable  drug. A partner may decide to pursue
an alternative strategy or alternative partners. A partner that has been granted
marketing  rights for a certain drug within a geographic area may fail to market
the drug  successfully.  Consequently,  Hybridon's current strategic alliance or
those it enters  into in the future may not be  scientifically  or  commercially
successful.  Hybridon may not able to negotiate advantageous strategic alliances
in the future.  The absence of, or failure of,  strategic  alliances  could harm
Hybridon's efforts to develop and commercialize its drugs.

HSP's Results May Be Lower than Currently Anticipated

            Through HSP,  Hybridon  manufactures  oligonucleotide  compounds for
sale to others.  The results of HSP will depend on the demand for and margins on
these drugs,  which may be lower than Hybridon  anticipates.  HSP's results will
also be  affected  by the  price and  availability  of raw  materials  and HSP's
production costs.

Hybridon Faces Intense  Competition,  and Hybridon's  Products Could Be Rendered
Obsolete;  Many of Hybridon's  Competitors Have Greater Resources and Experience
than Hybridon

            Many  companies  are  attempting  to  develop  drugs  similar  those
Hybridon  proposes to develop.  Some of these drugs are in clinical trials,  and
one has received FDA approval and is being  commercialized.  In addition,  there
are  other  drugs  available  for the  treatment  of many of the  diseases  that
Hybridon's  proposed  drugs  would  treat.  Any of these  drugs may  prove  more
effective  than those that  Hybridon  proposes to develop  and may gain  greater
market acceptance.

            Furthermore,  biotechnology and related pharmaceutical  technologies
have  undergone  and  continue  to be subject to rapid and  significant  change.
Hybridon expects that the technologies  associated with  biotechnology  research
and development will continue to develop rapidly.  Hybridon's future will depend
in large part on its ability to compete with these technologies.  Any compounds,
drugs or processes that Hybridon  develops may become  obsolete  before Hybridon
recovers the expenses incurred in developing those drugs.

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<PAGE>

            Many of Hybridon's competitors have substantially greater financial,
technical,  and human resources than Hybridon,  and have  significantly  greater
experience  than  Hybridon in  preclinical  studies,  clinical  trials,  seeking
regulatory approval of new drugs, and manufacturing and marketing new drugs.

Hybridon's  Manufacturing  Capability May Be Adversely Affected by Problems with
Suppliers

            Certain of the raw materials  that Hybridon  requires to manufacture
oligonucleotides  are  available  from only a few  suppliers,  namely those with
access to the  appropriate  patented  technology.  The  number of  suppliers  is
unlikely to increase in the near  future.  Hybridon may not be able to secure an
adequate  supply  of  these  materials  at an  acceptable  price.  Also,  due to
regulatory  restrictions  or other  problems,  Hybridon's  suppliers may fail to
provide to Hybridon drugs of acceptable  quality.  Such supplier  problems would
have an adverse  effect on Hybridon's  ability to  manufacture  oligonucleotides
cost-effectively.

Hybridon's Lack of Marketing  Experience  Could Adversely  Affect Its Ability to
Commercialize Its Drugs

            Direct marketing of any of Hybridon's proposed drugs would require a
substantial  marketing staff and sales force supported by a distribution system.
Given that Hybridon  currently has little  experience  in sales,  marketing,  or
distribution,  Hybridon might not be able to undertake  direct  marketing of its
drugs  in  a  cost-effective  manner.  The  alternative--co-marketing  or  other
licensing  arrangements--would  allow  Hybridon  to avoid the  significant  cost
involved in direct marketing,  but would make Hybridon reliant on the efforts of
others.

Hybridon Could Be Subject to Product  Liability Claims for Which It Is Not Fully
Insured

            Hybridon risks being the target of product liability claims alleging
that its drugs harm  subjects  or  patients.  Such  claims  could be asserted in
connection  with  Hybridon  drugs used in clinical  trials as well as those sold
commercially.  Hybridon is covered  against  such claims by a product  liability
insurance  policy  (subject  to  various  deductibles),  but such  policies  are
becoming  increasingly  expensive,  and  Hybridon  may not be  able to  maintain
sufficient  coverage  to protect  it from  incurring  significant  losses due to
product liability claims.

Hybridon Uses Hazardous  Materials,  and Could Be Held Liable For Damages in the
Event of Accidental Contamination or Injury

            Hybridon's  activities  involve  the  controlled  use  of  hazardous
chemicals,  viruses, and radioactive compounds.  Although Hybridon believes that
its safety  procedures for handling and disposing of such materials  comply with
the standards prescribed by federal,  state, and local regulations,  the risk of
accidental contamination or injury cannot be completely eliminated. In the event
of such an accident, Hybridon could be held liable for any damages that result.

Restrictions  on Third-Party  Reimbursement  Could Adversely  Affect  Hybridon's
Ability to Commercialize Its Drugs

            Hybridon's  ability to commercialize  drugs successfully will depend
in part on the extent to which  various  third  parties are willing to reimburse
patients for the costs of Hybridon's drugs and related  treatments.  These third
parties  include  government  authorities,  private health  insurers,  and other
organizations, such as health maintenance organizations.  Third-party payors are
increasingly  challenging the prices charged for medical  products and services.
Accordingly,  if less costly  drugs are  available,  third-party  payors may not
authorize or may limit  reimbursement  for  Hybridon's  drugs,  even if they are
safer or more effective  than the  alternatives.  In addition,  the trend toward
managed  healthcare  and  government  insurance  programs  could result in lower
prices and  reduced  demand for  Hybridon's  drugs.  Cost  containment  measures
instituted  by  healthcare  providers  and any general  healthcare  reform could
affect  Hybridon's  ability to sell drugs and may have a material adverse effect
on  Hybridon.  Hybridon  may be forced to reduce its prices;  this would in turn
adversely affect profitability.



                                        9



<PAGE>

            Hybridon  cannot predict what  additional  legislation or regulation
relating to the health care industry or third-party  coverage and  reimbursement
may be enacted in the  future,  or what effect such  legislation  or  regulation
might have on its business. In particular,  Hybridon may be forced to reduce its
prices; this would in turn adversely affect profitability.

The Market Price of the Securities Is Likely to Be Volatile

            The market price of the securities of  biotechnology  companies such
as Hybridon are highly  volatile.  The market price of Hybridon's  securities is
likely to be  influenced  by the results of  preclinical  studies  and  clinical
trials by Hybridon or its  competitors,  fluctuations  in  Hybridon's  operating
results,   announcements   by  Hybridon  or  its  competitors  of  technological
innovations  or new commercial  therapeutic  products,  changes in  governmental
regulation,  developments in patent or other  proprietary  rights of Hybridon or
its competitors, public concern as to the safety of drugs developed by Hybridon,
and general market conditions.

Hybridon's Ability to Utilize Its Net Operating Losses and Tax Credits Is Likely
to Be Severely Restricted

            Hybridon  has   substantial   net  operating  loss  and  tax  credit
carryforwards for federal income tax purposes.  These  carryforwards will expire
beginning on December 31, 2005. The Tax Reform Act of 1986 limits the annual use
of net operating loss and tax credit  carryforwards  following certain ownership
changes.  The securities  offerings conducted by Hybridon will severely restrict
Hybridon's  ability to utilize its net  operating  losses and tax credits in any
particular year.  Additionally,  because the U.S. tax laws limit the time during
which net operating  loss and tax credit  carryforwards  may be applied  against
future taxable income and tax liabilities,  respectively,  Hybridon may never be
fully able to use its net operating  loss and tax credits for federal income tax
purposes.

Hybridon May Be Adversely Affected by Year 2000 Compliance Related Problems

            As  has  been  widely   publicized,   many   computer   systems  and
microprocessors  are not programmed to  accommodate  dates beyond the year 1999.
Hybridon's  exposure  to this Y2K problem  comes not only from its own  internal
computer   systems   and   microprocessors,   but  also  from  the  systems  and
microprocessors  of its key suppliers,  including  utility companies and payroll
services.  While Hybridon  currently  believes that all of its internal  systems
will be Y2K  compliant  by the end of the third  quarter of 1999,  and is taking
appropriate  measures  to ensure that its  suppliers  are Y2K  compliant,  it is
nevertheless  possible  that  Y2K  problems  will  have  a  material  effect  on
Hybridon's  business.  See  "Management's  Discussion  and Analysis of Financial
Condition and Results of Operations--Year 2000."

Stock  Ownership  by  Hybridon's  Directors  and Officers May Delay or Prevent a
Change of Control

            Hybridon's  directors  and executive  officers and their  affiliates
beneficially own a significant percentage of Hybridon's outstanding Common Stock
and Convertible  Preferred  Stock. As a result,  these  stockholders,  if acting
together,  may have the ability to influence  the outcome of  corporate  actions
requiring  stockholder  approval.  This  concentration of ownership may have the
effect of delaying or preventing a change in control of Hybridon.

                           FORWARD-LOOKING STATEMENTS

            This  Prospectus  contains  forward-looking  statements  that do not
reflect historical facts, but instead reflect  Hybridon's current  expectations,
estimates,  and projections regarding its business.  Forward-looking  statements
can be found in the material  set forth under "Risk  Factors,"  "Business,"  and
"Management's  Discussion  and  Analysis of Financial  Condition  and Results of
Operations," and are characterized by use of words such as "believes,"  "plans,"
"expects," and "anticipates."  Forward-looking  statements are not guarantees of
future  performance,  and  necessarily  involve  risks  and  uncertainties,  and
Hybridon's  results  could  differ  materially  from  those  anticipated  in the
forward-looking statements contained in this Prospectus.


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<PAGE>

                                    BUSINESS

HYBRIDON

            Hybridon,  established  in 1989,  is a leader in the  discovery  and
development of genetic drugs.  These drugs are based on "antisense"  technology,
in that they use synthetic RNA and DNA, also called  oligonucleotides,  with the
aim of stopping or reducing the body's  production  of proteins that directly or
indirectly  cause a given disease.  Hybridon's  leadership  position is based on
following factors:

o    its access to  proprietary  technology  for the  chemical  modification  of
     oligonucleotides
o    its expertise in the efficient design and development of antisense drugs
o    innovations it has devised in the manufacture of oligonucleotides
o    its large-scale oligonucleotide  manufacturing facility
o    its expertise in the manufacture of oligonucleotides  with diverse chemical
     modifications

            Hybridon has developed and owns antisense  technology  that includes
important new medicinal  chemistries  (relating to the design and manufacture of
new medicinal  compounds),  analytical  chemistry (relating to the detection and
identification  of  compounds  inside  and out of the body),  and  manufacturing
technology.

TECHNOLOGY OVERVIEW

Introduction

            The heart, brain, liver, and other organs in the human body function
together to support life.  Each  microscopic  cell within these organs  produces
proteins that affect how that cell  functions  within its organ,  and ultimately
how efficiently each organ functions within the body.  Almost all human diseases
are caused by abnormal  production or performance of proteins within  individual
cells.  In some  instances,  cell  proteins  act  directly to cause or support a
disease.  In other instances,  cell proteins  interfere with other proteins that
prevent or combat  disease.  Traditional  drugs are  designed to  interact  with
protein molecules that cause or support  diseases.  Antisense drugs are designed
to work at an earlier stage in that they are designed to stop the  production of
disease-causing or disease-supporting proteins.

            The  information  that  controls a cell's  production  of a specific
protein is contained in the gene relating to that protein.  Each gene is made up
of two intertwined strands of DNA that form a structure called a "double helix."
Each  strand of DNA  consists of a string of  individual  DNA  building  blocks,
called nucleotides,  arranged in a specific sequence.  One of the paired strands
contains the information that directs the composition of a specific protein, and
is called the  "coding"  strand.  The other  strand,  the  "non-coding"  strand,
contains a different but complementary  sequence of nucleotides.  Each strand is
made of linked molecules,  known as the "backbone," and attached to the backbone
are  molecules  known as  "bases."  It is the  sequence  of  bases,  and not the
backbone,   that  contains   genetic   information.   When   Hybridon   modifies
oligonucleotides chemically, it modifies the backbone, and not the bases.

            The full  complement  of human genes,  known as the human  "genome,"
consists of over 100,000 genes and contains the information  required to produce
all human proteins. A copy of the complete human genome is present in each cell,
and  each  cell  makes  proteins  based on its copy of the  genome.  Cells  make
proteins in a two-stage process. First, the cell creates a molecule of messenger
RNA  consisting of a string of nucleotides  in a sequence  complementary  to the
sequence of the coding  strand of DNA.  This is called the "sense"  sequence.  A
sequence that is  complementary  to the sense sequence is called the "antisense"
sequence.  The cell then produces proteins based on the information contained in
the messenger  RNA. The number of copies of messenger RNA the cell produces will
affect how many copies of a given protein it produces.

            A normal cell  produces a given set of normal  proteins in the right
amount for the body to function properly. A diseased cell produces inappropriate
or mutant  proteins,  or produces  the wrong amount of normal  proteins.  A cell
produces mutant proteins when its DNA changes,  either through  mutation,  as in
many types of cancer cells, or by infection with a virus.


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<PAGE>

Conventional Drugs

            Most drugs are chemicals  that stimulate or suppress the function of
a particular molecule, usually a protein, with tolerable side effects. Most side
effects  arise when a drug  interacts  with  proteins  in addition to the target
protein.  Generally,  the fewer other proteins a drug interacts  with, the fewer
the side effects.

            Conventional drugs generally aim to bind only two or three points of
the target molecule.  Frequently,  however,  sites on other non-target molecules
resemble the target binding site enough to permit the conventional  drug to bind
to some  degree to those  non-target  molecules.  This lack of  selectivity  can
result in unwanted side effects, potentially leading to decreased effectiveness.

            A further  characteristic  of conventional  drugs is that developing
them is a time-consuming  and expensive  process,  as for every compound that is
found  to be  effective  and  have  tolerable  side  effects,  thousands  may be
investigated and rejected.

Antisense Drugs

            An oligonucleotide with a sequence exactly  complementary to that of
the messenger RNA of a specific  gene can bind to and  inactivate  the messenger
RNA,  thereby  decreasing or eliminating  the production of  disease-causing  or
disease-supporting  proteins.  Antisense  technology  involves  the  design  and
synthesis  of such  oligonucleotides.  Hybridon  believes  that  drugs  based on
antisense technology may be more effective, cause fewer side effects, and have a
greater range of applications  than  conventional  drugs because antisense drugs
are designed to intervene in the  production of proteins,  rather than after the
proteins are made, and in a highly specific fashion.

            Advances in mapping the human genome,  including  work  conducted by
academic institutions,  biotechnology companies,  and pharmaceutical  companies,
have allowed  many targets for  antisense  drugs to be  identified.  Once a gene
associated  with  a  disease-associated  protein  is  identified,  an  antisense
oligonucleotide  can  be  designed,  and  the  pharmaceutical  effects  of  that
oligonucleotide  can be improved by chemical  modification.  Chemically-modified
oligonucleotides can be composed of DNA, RNA, or a combination of the two.

            Because the nucleotide sequence of a  chemically-modified  antisense
oligonucleotide  is complementary to its target sequence on the messenger RNA of
a given gene, the antisense oligonucleotide forms a large number of bonds at the
target site,  typically  between 40 and 60. This allows it to form a strong bond
with the messenger  RNA. A few  identical  messenger RNA molecules can cause the
cell  to  produce  many  copies  of  a  protein;   similarly,  a  few  identical
chemically-modified  antisense  oligonucleotides can stop this process.  This is
due in part to an enzyme called RNase H that can destroy  messenger RNA bound to
an oligonucleotide  without destroying the oligonucleotide  itself, thus freeing
the  oligonucleotide to bind with, and cause the destruction of, other messenger
RNA molecules.  This process is generally  known as catalytic  activity.  All of
Hybridon's  drugs are designed to take advantage of this  catalytic  activity so
that a relatively  small number of antisense  molecules can effectively  inhibit
production of disease-associated proteins.

HYBRIDON ANTISENSE TECHNOLOGY

            Hybridon's  antisense chemistry builds on the pioneering work in the
antisense  field  begun  in the  1970s  by Dr.  Paul  C.  Zamecnik,  a  founder,
consultant and director of Hybridon.  The  development  of Hybridon's  antisense
chemistry has been directed by Dr. Sudhir Agrawal,  Hybridon's  Chief Scientific
Officer,  and is based on what is referred to in this  Prospectus  as  "advanced
chemistries,"  namely  Hybridon's  ability to alter the  chemical  makeup of the
oligonucleotide  backbone in a manner that makes oligonucleotides safer and more
stable without  adversely  affecting their ability to promote the destruction of
messenger RNA.

            Medicinal  Chemistries.  Hybridon's first antisense drug, GEM(R) 91,
was based on  first-generation  phosphorothioate  chemistry,  which  altered the
naturally-occurring,  or native,  form of  oligonucleotides by replacing certain
phosphorus  atoms in the backbone with sulfur  atoms.  GEM(R) 91 was more stable
than native


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<PAGE>

DNA,  but was still able to trigger the action of RNaseH,  leading to  catalytic
activity.  However, there were side effects caused by the administration of this
modified DNA into the body. In particular,  in the last clinical trial of GEM(R)
91 three of the nine  patients  treated  experienced  unacceptable  decreases in
platelet counts, which increased the possibility of uncontrolled  bleeding. As a
result, Hybridon discontinued the GEM(R) 91 program. Hybridon has, however, used
the information gained from the human clinical trials of GEM(R) 91 to design its
advanced oligonucleotide chemistries.

            Hybridon's scientists have designed and made over twenty families of
advanced  oligonucleotide  chemistries,  including  DNA/RNA  combinations,  also
called hybrid or mixed backbone  chemistries.  Hybridon  believes that antisense
compounds based on these advanced chemistries will show favorable pharmaceutical
characteristics  that will  significantly  increase their potential  therapeutic
value.   These   compounds   are   likely  to  have  the   following   desirable
characteristics:

o    catalytic activity
o    fewer side effects
o    greater stability in the body,  thereby  permitting a patient to take doses
     less frequently
o    greater potency, thereby permitting a patient to take lower doses
o    potential  for multiple  routes of  administration  (such as by  injection,
     orally, or topically)

            Manufacturing  Technology.  Hybridon's expertise in the synthesis of
chemically  modified  oligonucleotides  has  served  as  the  foundation  of its
manufacturing  technology  and  know-how.  Hybridon  has  developed  proprietary
technology, including equipment, to increase the purity of its oligonucleotides,
make the production  process more  efficient,  increase the scale of production,
and significantly reduce the cost of oligonucleotide-based drugs.

            Proprietary  Analytical Tools.  Hybridon has established  analytical
tools and processes that enable it to test the purity of  oligonucleotides  more
quickly  and  accurately  than  would be  feasible  using  traditional  methods.
Hybridon uses the resulting information to improve quality control, to assist it
in  complying  with  regulatory  requirements,  and to  monitor  absorption  and
stability of its drugs in preclinical and clinical trials.

            Regulatory  Know-How.  Hybridon drug  development and  manufacturing
personnel  have  extensive  experience  in  working  with the FDA and other drug
regulatory  agencies in an efficient and cost-effective  manner.  Hybridon often
assists  customers  of  Hybridon   Specialty   Products,   Hybridon's   contract
manufacturing  division, also called "HSP," by contributing essential components
of their submissions to the FDA.

HYBRIDON DRUG DEVELOPMENT AND DISCOVERY PROGRAMS

The Drug Development and Approval Process

            The  process  of  taking a  compound  from the  laboratory  to human
patients generally takes 10 to 15 years. This process is extremely expensive and
is  rigorously  regulated by  governmental  agencies,  including,  in the United
States, the Food and Drug Administration, or the "FDA." Each drug must undergo a
series  of  trials  (preclinical  and  clinical)  before  the FDA will  consider
approving it for commercial sale. The FDA or any company  conducting drug trials
can  discontinue  those  trials at any time if it feels that  patients are being
exposed to an  unacceptable  health risk or if there is not enough evidence that
the drug is effective.  The FDA may also require a company to provide additional
information or conduct  additional tests before it will permit a drug to proceed
from one phase of trials to the next.

            The phases of preclinical and clinical trials are described below.

o    Preclinical  Studies.  Preclinical  trials  involve  the testing of a given
     compound  in  animals  to provide  data on the  activity  and safety of the
     compound before the compound is administered to humans.

o    Investigational  New  Drug  Application.  If the  data  from  research  and
     preclinical trials are promising,  the company will file an Investigational
     New Drug Application, or "IND," with the FDA. The IND


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<PAGE>

     contains  the results of the  preclinical  trials and the  protocol for the
     first  clinical  trial.  The IND  becomes  active in 30 days unless the FDA
     disapproves  it or requires  additional  information.  Once the IND becomes
     active, the company can begin clinical trials in humans.

o    Phase I Clinical  Trials.  In Phase I trials,  the drug is given to a small
     group of healthy individuals or patients with the disease. These trials are
     designed to produce data on the drug's  safety,  the maximum safe dose, and
     how the drug is absorbed, distributed, metabolized, and excreted over time.
     In some  cases,  Phase I trials  can give an early  indication  of a drug's
     effectiveness.  A limited Phase I trial is sometimes called a Pilot Phase I
     trial.

o    Phase I/II  Clinical  Trials.  In Phase I/II  trials,  the drug is given to
     patients  with  the  diseases  to  evaluate  safety  and to  get  an  early
     indication of a drug's  effectiveness.  This type of trial is commonly used
     in the evaluation of oncology drugs.

o    Phase II Clinical Trials. In Phase II trials, the drug is given to a larger
     group of patients  with the disease for purposes of  evaluating  the drug's
     effectiveness   and  side  effects  at  varying   doses  and  schedules  of
     administration  and thereby  determining  the optimal dose and schedule for
     the larger Phase III trials that follow.

o    Phase III Clinical  Trials.  These trials  generally have a large number of
     patients. The primary purpose of a Phase III trial is to confirm the drug's
     effectiveness and produce additional information on side effects.

o    New Drug Application.  Once Phase III trials are complete, the company will
     file a New Drug  Application,  or "NDA," with the FDA. The NDA contains all
     of the information  gathered from the Phase I, II and III trials.  Based on
     the FDA's  review of the NDA,  the FDA may approve the drug for  commercial
     sale. The FDA may deny an NDA if the applicable regulatory requirements are
     not met. The FDA may also require additional tests before approving an NDA.
     Even after  approval by the FDA, the company must file  additional  reports
     about the drug with the FDA from time to time. The FDA may withdraw product
     approvals if a company fails to comply with ongoing regulatory standards or
     if problems occur after a company starts marketing a drug.

o    Accelerated Approval.  The FDA is authorized to grant accelerated review to
     NDAs for drugs that are  intended to treat  persons with  debilitating  and
     life-threatening illnesses,  especially if no satisfactory alternatives are
     available. The more severe the disease, the more likely it is that the drug
     will  qualify  for  accelerated  review.  If a new drug is  approved  after
     accelerated  review,  the FDA may require the company that filed the NDA to
     conduct  specific  post-marketing  studies  regarding  the  drug's  safety,
     benefits and optimal use.

     The regulatory  process in other countries is generally similar to the U.S.
     regulatory process.

Hybridon Drug Development and Discovery Programs

            Hybridon is focusing its drug  development and discovery  efforts on
developing  antisense  compounds  for the  treatment  of diseases in three major
therapeutic areas: cancer, viral infections, and diseases of the eye.

            Hybridon believes there are significant additional opportunities for
the use of  antisense,  particularly  in the  treatment  of cancer.  Compared to
conventional drugs, antisense may provide:

o    more specific therapy

o    more rapid development of drugs targeting  newly-discovered  cancer-related
     proteins

o    fewer toxic side effects,  thereby  allowing repeat and long-term  therapy,
     either  alone  or in  combination  with  other  cancer  therapies  (such as
     radiation or chemotherapy)


                                       14



<PAGE>

o    when used in combination  therapy,  therapeutic effects that complement the
     benefits of conventional drugs

For these reasons,  Hybridon is exploring new antisense  targets relevant to the
treatment of cancer.

CLINICAL PROGRAMS

            Hybridon  has  conducted   clinical   trials  with  antisense  drugs
targeting the following  diseases.  Hybridon is seeking partners for each of its
compounds in clinical development.

Cancer

            Unlike normal human cells,  cancer cells grow in an uncontrolled and
harmful  manner.  The  protein  molecule  protein  kinase A, or "PKA,"  has been
implicated in the formation and growth of various solid tumors, including colon,
ovarian,  breast,  and lung tumors.  There are two kinds of PKA. It is normal to
find  type I in  developing  fetuses,  but  abnormal  to find it in  adults.  By
contrast,  PKA type II is found in, and is  necessary  to the health of,  normal
adults.  Certain  cancer  cells  produce  PKA  type I in  adults.  Hybridon  has
developed a cancer drug,  GEM(R) 231, that is designed to reduce the  production
of the harmful PKA type I without  interfering  with the  production of PKA type
II. Current drug candidates based on conventional  mechanisms have  unacceptable
side effects.

            Hybridon  has  conducted a Phase I clinical  trial to  evaluate  the
safety of GEM(R) 231 at multiple doses, and has found that patients  tolerate it
well.  This trial  explored  the maximum  tolerated  dose of GEM(R) 231 for both
single doses and multiple doses,  and even high doses of GEM(R) 231 did not show
the side effects  normally seen with current cancer  treatments.  This trial was
not conducted for the purpose of evaluating the efficiency of GEM(R) 231.

            Hybridon is currently conducting  additional studies with GEM(R) 231
in patients  with solid tumors that had not been cured by prior  therapy.  These
studies  include a pilot  Phase II trial and a Phase I/II  trial.  In  addition,
Hybridon has begun the first in a series of Phase I/II trials treating  patients
with  solid  tumors  with  GEM(R)  231 in  combination  with  other  anti-cancer
therapies, such as Taxol(R), Taxotere(R) or radiation therapy.

HIV-1 and AIDS

            Acquired  Immune  Deficiency  Syndrome,  or  "AIDS,"  is  caused  by
infection with the Type 1 Human Immunodeficiency Virus, or "HIV-1," and leads to
severe,  life-threatening  impairment of the immune system. AIDS therapy using a
combination of drugs has resulted in decreased rates of death and improvement in
the quality of life for patients who are  HIV-positive  or have AIDS.  There are
however,  increasing  reports that this therapy may be failing to give sustained
clinical  benefit.  Hybridon  believes  this  underscores  the need for new AIDS
therapies.

            Hybridon has  completed a Pilot Phase I clinical  study in Europe of
GEM(R) 92,  Hybridon's  advanced  chemistry  compound for the treatment of HIV-1
infection  and AIDS.  This study was designed to explore the safety of GEM(R) 92
and to provide  information on its  absorption  after oral dosing and injection.
The patients tolerated well all doses given in the pilot study. Further,  GEM(R)
92 was  detected in the blood after both oral dosing and  injection,  suggesting
that it may be possible to develop GEM(R) 92 as an oral drug.  Hybridon believes
this was the first study of the oral  administration of an antisense molecule to
humans. In in-vitro studies, beneficial effects were observed when GEM(R) 92 was
used in  combination  with several  marketed AIDS drugs.  Importantly,  both its
medicinal  approach and genetic target are unique, in that no antisense drug has
been approved for the  treatment of AIDS,  and no other drug has the same target
on the HIV-1 genome.

PRECLINICAL PROGRAMS

Hybridon has also conducted preclinical studies in the following areas.


                                       15



<PAGE>

<TABLE>
<CAPTION>

                                                     Primary Therapeutic
Target                                               Indication(s)                            Status
- ----------------------------------------------------------------------------------------------------------------------------------

<S>                                                   <C>                                        <C>
MDM2 (a protein involved in                          Cancer                                   Ongoing; part of the Searle
programmed cell death)                                                                        collaboration

Vascular Endothelial Growth                          Cancer                                   Seeking partner
Factor (a protein that can cause
abnormal formation of new blood                      Retinopathies (e.g.                      Seeking partner
vessels)                                             macular degeneration and
                                                     diabetic retinopathy)

                                                     Psoriasis                                Seeking partner

Hepatitis C Virus                                    Hepatitis C (which can                   Seeking partner
                                                     lead to liver cancer)
- ----------------------------------------------------------------------------------------------------------------------------------
</TABLE>


HYBRIDON SPINOUTS

            Hybridon has used multiple  strategies to fund  applications  of its
antisense technology that it cannot develop at present without external funding.
Hybridon has used one such  strategy,  formation of a spinout  company,  to form
MethylGene, Inc. and OriGenix Technologies Inc. for the continued development of
certain product candidates.

MethylGene, Inc.

            In 1996, Hybridon and three Canadian institutional  investors formed
MethylGene.  Hybridon owns approximately 30% of MethylGene. Hybridon has granted
exclusive worldwide licenses and sublicenses to MethylGene to develop and market
(1)  antisense  compounds to inhibit the protein DNA  methyltransferase  for the
treatment of any disease, (2) other methods of inhibiting DNA  methyltransferase
for the treatment of any disease,  and (3) antisense  compounds to inhibit up to
two additional targets for the treatment of cancers. Research has shown that DNA
methyltransferase,   a  protein,   is  overproduced  in  some  tumors,  such  as
non-small-cell lung cancer,  colon cancer, and breast cancer tumors.  MethylGene
is  obligated  to  purchase   from   Hybridon  at  specified   prices  all  bulk
oligonucleotides  that  MethylGene  requires.  Hybridon is also  performing drug
development and other services for MethylGene.

            The Canadian  investors who invested in MethylGene have the right to
exchange all (but not less than all) of the shares of stock in  MethylGene  that
they  initially  purchased  for shares of Hybridon  Common Stock on the basis of
37.5 MethylGene shares (for which they paid  approximately  U.S. $56.25) for one
share of Hybridon  Common Stock (subject to adjustment  for stock splits,  stock
dividends and the like). This option expires no later than 2001.

            MethylGene  submitted  INDs  in the  United  States  and  Canada  in
December  1998 and in May 1999  commenced  Phase I clinical  trials of its first
compound, MG98, for the treatment of cancer.

OriGenix Technologies Inc.

            In January 1999, Hybridon and three Canadian institutional investors
formed  OriGenix to develop and market  drugs for the  treatment  of  infectious
diseases,  with an initial focus on viral diseases.  Hybridon owns approximately
49%  of  OriGenix.  If  OriGenix  satisfies  certain  conditions,  the  Canadian
investors  are  required to make an  additional  investment,  which would reduce
Hybridon's ownership interest in OriGenix to 40%.


                                       16



<PAGE>

            Hybridon has granted to OriGenix  worldwide  exclusive  licenses and
sublicenses to antisense  technology  developed by Hybridon for the treatment of
human papillomavirus,  or "HPV," and hepatitis B virus infections. HPV infection
can cause a variety of warts,  including benign genital warts. HPV infection can
also lead to cervical cancer. Hepatitis B infections can lead to liver cirrhosis
and cancer of the liver.  OriGenix may in the future negotiate with Hybridon for
licenses or sublicenses relating to additional targets. In addition, OriGenix is
obligated   to   purchase   from   Hybridon   at   specified   prices  all  bulk
oligonucleotides  it requires.  Hybridon  anticipates  that it will perform drug
development and other services for OriGenix.

CORPORATE COLLABORATIONS

            An important part of Hybridon's  business  strategy is to enter into
research  and  development   collaborations,   licensing  agreements,  or  other
strategic  alliances with others,  primarily  biotechnology  and  pharmaceutical
corporations,  to develop  certain  products.  Hybridon  intends to proceed with
Phase II clinical  trials of its cancer drug,  GEM(R) 231.  Otherwise,  Hybridon
does not anticipate  proceeding  with any of its other clinical  programs beyond
their current stages of development  without a collaborative  arrangement with a
corporate  partner.  Hybridon is currently a party to  corporate  collaborations
with Searle and Medtronic.  Hybridon expects to retain the rights to manufacture
many of the  products it may license  pursuant  to its  existing  and any future
collaborations.

G.D. Searle & Co.

            In January 1996,  Hybridon and Searle  entered into a  collaboration
for research and development of therapeutic  antisense compounds in the areas of
inflammation/immunomodulation. The collaboration agreement, as modified in April
1998,  provides  that Searle may also select  specific  targets in the fields of
cancer and cardiovascular disease.

            Hybridon   and  Searle  are   currently   conducting   research  and
development  relating to compounds targeting MDM2, a protein that is involved in
programmed  cell  death  and that may play a role in  cancer.  In this  project,
Searle is funding  certain  research and  development  efforts at Hybridon,  and
Searle and Hybridon have committed  personnel to the collaboration.  The initial
phase of research  and  development  activities  will be  conducted  through the
earlier of (1) the  achievement of certain  milestones and (2) January 31, 2000,
subject to early  termination  by Searle.  The  parties may agree to extend this
collaboration.

            In addition,  Searle may  designate up to six  additional  molecular
targets in the specified fields on terms substantially consistent with the terms
applicable to the initial targets.  To do so, it must pay specified cash amounts
(in addition to specific research  payments relating to each additional  target)
and  purchase  additional  Common  Stock from  Hybridon (at the then fair market
value), with the total payment per additional target equalling  $10,000,000.  If
Searle designates all of the additional targets,  Searle will pay $24,000,000 in
cash and purchase $36,000,000 of equity. If Searle has not designated all of the
additional targets by the time any drug relating to the initial molecular target
reaches a certain stage of preclinical  development,  Searle must purchase up to
an  additional  $10,000,000  of Common Stock (at the then fair market  value) in
order to keep its right to designate any of the additional targets. This payment
will be credited against the equity  investment  payments made by Searle for any
additional targets it designates in the future.

            Searle has exclusive  rights to  commercialize  any drugs  resulting
from the  collaboration.  If Searle elects to commercialize a drug,  Searle will
fund and perform  preclinical  studies and clinical trials of that drug and will
be responsible  for obtaining  regulatory  approvals for, and marketing of, that
drug.  Hybridon  has agreed to perform  certain  research and  development  work
exclusively with Searle. In addition, for each drug candidate,  Searle must make
payments to Hybridon of up to  $10,000,000  upon the  achievement of development
milestones.  Hybridon  will  also be  entitled  to  royalties  from net sales of
products  commercialized as a result of the  collaboration.  As long as Hybridon
satisfies  certain  requirements  relating to its  manufacturing  capacities and
capabilities,  Hybridon  will retain  manufacturing  rights,  and Searle will be
required to purchase its requirements of bulk  oligonucleotides from Hybridon on
an exclusive  basis at specified  prices.  Upon a change in control of Hybridon,
Searle  would  have the  right to  terminate  Hybridon's  manufacturing  rights,
although the royalty payable to Hybridon from net sales would be increased.


                                       17



<PAGE>

            If Searle  designates all of the  additional  targets or if Hybridon
fails to satisfy certain requirements  relating to its manufacturing  capacities
and capabilities, Searle will have the right to require Hybridon to form a joint
venture  with Searle for the  development  and  commercialization  of  antisense
therapeutic  products in the area of  inflammation/immunomodulation  (other than
products  relating to targets that have already  been  designated  by Searle) to
which  Searle  will  contribute  $50,000,000  in cash and  certain  intellectual
property rights. Hybridon will also contribute certain intellectual property and
technology  and,  if the fair  market  value  of that  technology  is less  than
$50,000,000,  Hybridon will, at its discretion, either contribute the difference
in cash or have its share of the first profits of the joint  venture  reduced by
the amount of that  difference.  Hybridon  and Searle  would each own 50% of the
joint venture, although Searle's ownership interest could increase to as much as
75% if the joint venture is established because of Hybridon's failure to satisfy
the requirements relating to its manufacturing capacities and capabilities.

            Pursuant to the collaboration,  Searle also purchased 200,000 shares
of Common Stock in Hybridon's initial public offering.

Medtronic, Inc.

            In May 1994,  Hybridon  and  Medtronic  agreed to test a device  for
delivering   Hybridon's   antisense   oligonucleotides   for  the  treatment  of
Alzheimer's disease. The agreement provides that Hybridon is responsible for the
development  of, and will hold all rights to, any drug  developed as a result of
this agreement and Medtronic is  responsible  for the  development  of, and will
hold all rights to, any delivery system developed as a result of this agreement.
The parties may agree to extend this  collaboration  to other  neurodegenerative
disease targets.  Hybridon is not currently conducting any activities under this
agreement.

            As part of  their  collaboration,  Medtronic  purchased  a total  of
131,667 shares of Hybridon Common Stock.

HYBRIDON SPECIALTY PRODUCTS

            In 1996, Hybridon formed Hybridon Specialty  Products,  or "HSP," to
manufacture  oligonucleotide compounds both for Hybridon's internal use, for use
by its colloborators  and for sale to third parties.  Hybridon believes that the
current interest in genetic medicine or drugs based on genetic  information will
continue,  and even  increase,  as the potential of these  technologies  for the
development of new classes of drugs becomes more widely understood,  and that as
a result demand for oligonucleotide compounds will increase. Hybridon's strategy
is to position HSP to take advantage of this increased  demand.  There can be no
assurance  that this strategy will be successful or that demand will increase as
anticipated.  HSP is, however, attempting to minimize this risk by manufacturing
oligonucleotides for many applications,  at different stages of development. HSP
is  currently  manufacturing   oligonucleotides  for  genomic,   diagnostic  and
therapeutic  applications,  and Hybridon believes HSP's customers are developing
over 20  oligonucleotide  drugs,  with at  least  eight  currently  in  clinical
studies.

            HSP manufactures  oligonucleotides at its 36,000-square-foot  leased
facility,  which  Hybridon  believes is the only facility  currently  capable of
manufacturing  oligonucleotides  on a large  scale.  HSP first  began  producing
oligonucleotide   compounds   for  sale  in  June  1996  and  had   revenues  of
approximately  $1.1  million in 1996,  $1.9  million in 1997 and $2.8 million in
1998.  HSP's principal  customers in 1998 included Genta  Incorporated,  LaJolla
Pharmaceuticals, Inc. and MethylGene, Inc.

            HSP has developed a manufacturing  technology platform that combines
multiple  methods to improve the  production  process and increase the amount of
compounds produced in a single batch, thereby permitting economies of scale. HSP
has developed two separate machines,  called  synthesizers,  for the large-scale
synthesis of  oligonucleotides.  One of these machines was developed by Hybridon
alone and the other in collaboration with Pharmacia  Biotech.  Pharmacia has the
right  to make and  sell  synthesizers  based  on the  design  developed  in the
collaboration but must also pay Hybridon  royalties.  Hybridon believes that its
synthesizers  are  the  first  commercial-scale   oligonucleotide   synthesizers
designed  for  advanced  oligonucleotide   chemistries.  In  addition,  HSP  has
developed  purification  processes that use water in place of chemical solvents,
thereby decreasing the


                                       18



<PAGE>

impact of the process on the  environment  and  permitting  HSP to purify  large
quantities  of  oligonucleotides.  HSP has also  developed  processes and unique
chemicals  used in the  process,  which  HSP  believes  may  further  lower  its
production costs.

            In  1996,  Hybridon  entered  into  a  four-year  sales  and  supply
agreement  with the Applied  Biosystems  Division of  Perkin-Elmer,  pursuant to
which  Perkin-Elmer  agreed to refer  potential  customers  to HSP, and Hybridon
agreed to purchase  certain raw materials from  Perkin-Elmer for the manufacture
of  oligonucleotides  sold to  those  customers.  Hybridon  is  required  to pay
Perkin-Elmer  a  percentage  of the  sales  price  paid by those  customers.  In
addition,  Perkin-Elmer  licensed  to  Hybridon  its  oligonucleotide  synthesis
patents.

            HSP is targeting three market areas for oligonucleotides:  antisense
therapeutics,  non-antisense  therapeutics,  and  diagnostic/genomic DNA probes,
which are  oligonucleotides  designed to detect the presence of specific  genes.
Within each area there is a large number of potential products. HSP is currently
manufacturing  oligonucleotides  for  customers  in each of these  three  market
areas.

            The  production of  oligonucleotides  is similar in many respects to
the chemical synthesis used to produce conventional drugs. However,  unlike many
conventional  drugs,  one  can  with  the  same  chemical  building  blocks  and
essentially  the same  manufacturing  processes  and  equipment  make  different
antisense compounds for treating different diseases.  As a result, the knowledge
and experience that HSP obtains  manufacturing one oligonucleotide  compound can
be applied to the manufacture of other oligonucleotide  compounds.  Furthermore,
since several  different  oligonucleotide  compounds can be  manufactured in one
facility,  Hybridon  anticipates  that HSP will have the ability to  manufacture
multiple marketed oligonucleotide-based drugs without having to build a separate
plant for each such compound.

            In order to meet Hybridon's  needs and satisfy  outside demand,  HSP
may need to increase its manufacturing  capacity by adding more  oligonucleotide
synthesizers.  In addition, in order for Hybridon to successfully  commercialize
its drugs or for HSP to achieve a satisfactory  profit on sales, HSP may need to
reduce its production costs further.

            Hybridon    believes    that   it   is    currently    manufacturing
oligonucleotides  according to FDA Good Manufacturing  Practices,  or "GMP." The
FDA has not formally  reviewed HSP's facility and  procedures,  and Hybridon may
need to revise those  procedures  in the future as production  increases.  Since
1996, HSP has undergone multiple significant audits for GMP compliance conducted
by biotechnology and pharmaceutical companies. No significant deficits have been
identified.  In addition,  in 1997, HSP was one of two  biotechnology  companies
chosen to participate in the FDA's  Biotechnology PAI Pilot Initiative,  a pilot
program that allows FDA  regulatory  officials to provide advice to the selected
companies on  compliance  with FDA  standards  before they submit drug  approval
filings.  The FDA would have informed Hybridon of any substantial  issues if any
had arisen.

MARKETING STRATEGY

            Hybridon plans to market the drugs it is developing either directly,
using its own sales force, or through co-marketing,  licensing,  distribution or
similar  arrangements with other  pharmaceutical  and  biotechnology  companies,
particularly    if   the    products    are   intended   to   serve   a   large,
geographically-diverse patient population.

            Direct  marketing  of any of its  proposed  drugs  would  require  a
substantial  marketing staff and sales force supported by a distribution system.
Co-marketing or other  arrangements  with other  pharmaceutical or biotechnology
companies would allow Hybridon to avoid the significant  cost involved in direct
marketing,  but would  make  Hybridon  reliant on the  efforts of others.  While
Hybridon  has  developed  general  marketing  strategies,  it has not  begun  to
implement any of these strategies.



                                       19



<PAGE>

ACADEMIC AND RESEARCH COLLABORATIONS

            Hybridon  has  entered  into  a  number  of  collaborative  research
relationships  with  independent  researchers and leading  academic and research
institutions and U.S. government agencies,  including the National Institutes of
Health,  or "NIH." Such  research  relationships  allow  Hybridon to augment its
internal  research  capabilities  and obtain access to specialized  knowledge or
expertise.

            In general,  Hybridon's  collaborative  research  agreements require
Hybridon  to pay  various  amounts to support  the  research.  Hybridon  usually
provides the  oligonucleotides,  which the  collaborator  then tests.  If in the
course of conducting  research  under its agreement with Hybridon a collaborator
solely or jointly with Hybridon,  creates any invention,  Hybridon generally has
an option to negotiate an exclusive,  worldwide,  royalty-bearing license to the
invention.  Inventions  developed solely by Hybridon's  scientists in connection
with a collaborative  relationship  generally are owned exclusively by Hybridon.
Most of these collaborative  agreements are nonexclusive and can be cancelled on
short notice.

            Since July 1997, as part of its restructuring,  Hybridon has allowed
a number of its collaborative  research  agreements to expire and has terminated
certain others,  but has maintained  those that it believes  support its current
drug discovery and development programs.

DRUG DEVELOPMENT SERVICES

            Hybridon's Drug Development  Department has experience in the design
and conduct of  preclinical  and clinical  trials and has prepared and submitted
reports and other  regulatory  documents in connection  with the three  Hybridon
advanced  chemistry  antisense  compounds  that have entered  clinical  studies.
Pursuant to a contract with MethylGene,  Hybridon's Drug Development  Department
has also used its expertise to help design and monitor the preclinical trials of
MethylGene's  antisense compound,  MG98, that led to MethylGene's  submission of
Investigational  New Drug,  or "IND,"  applications  in  Canada  and the  United
States. MethylGene compensated Hybridon for these services.  Hybridon expects to
perform similar services for OriGenix.

PATENTS, TRADE SECRETS, AND LICENSES

            Hybridon's  success  will depend to a large extent on its ability to
(1) obtain U.S. and foreign patent protection for drug candidates and processes,
(2) preserve trade secrets,  and (3) operate without  infringing the proprietary
rights of third parties.

            Hybridon's  policy  is  to  file  patent   applications  to  protect
technology,   inventions  and  improvements  that  it  considers   important  to
development of its business,  and to obtain licenses to other patents that could
help Hybridon maintain or enhance its competitive position. As of June 15, 1999,
Hybridon owned or  exclusively  licensed in excess of 90 U.S. and foreign issued
and allowed  patents,  of which 72 are U.S.  patents,  and 63 other U.S.  and 99
other foreign patent applications.  These patents and applications cover various
chemically-advanced   oligonucleotides,    target   sequences,   oligonucleotide
products, methods for making and purifying oligonucleotides, analytical methods,
and methods for antisense  treatment of various diseases.  The patents expire on
dates ranging from 2006 to 2015.

            Hybridon is the  worldwide  exclusive  licensee  under  several U.S.
issued   patents  or  allowed  patent   applications   owned  by  University  of
Massachusetts  Medical Center,  or "UMMC"  (formerly the Worcester  Foundation),
relating to oligonucleotides and hybrid or mixed backbone  chemistries.  Many of
these patents and patent  applications have corresponding  patents issued by, or
corresponding patent applications on file in, other major industrial  countries.
One of the issued U.S.  patents  and one of the issued  European  patents  cover
antisense  oligonucleotides  as new  compositions  of matter  for  stopping  the
replication  of  HIV.  Coverage  of  the  other  issued  U.S.  patents  includes
composition and use of oligonucleotides  based on advanced chemistries,  methods
of oligonucleotide production,  composition of certain modified oligonucleotides
that are  useful  for  diagnostic  tests or assays,  and  methods  of  purifying
oligonucleotides.  The UMMC patents licensed to Hybridon expire at various dates
starting in 2006.


                                       20



<PAGE>

            Hybridon is the  exclusive  licensee  under  various  other U.S. and
foreign patents and patent applications,  including two U.S. patent applications
owned   by   McGill   University   relating   to   oligonucleotides    and   DNA
methyltransferase.  Hybridon and Massachusetts  General Hospital jointly own one
issued  U.S.  patent  applicable  to  Alzheimer's  disease.  Hybridon  holds  an
exclusive  license to  Massachusetts  General  Hospital's  interests  under this
patent.

            Hybridon is a nonexclusive  licensee of certain  patents held by the
NIH relating to oligonucleotide phosphorothioates and is a nonexclusive licensee
of an NIH patent covering the phosphorothiolation of oligonucleotides. The field
of each of these licenses extends to a wide variety of genetic targets.

            The U.S.  Patent  and  Trademark  Office,  or  "PTO,"  has  informed
Hybridon that certain patent applications  exclusively licensed by Hybridon from
UMMC have been submitted to the Board of Patent Appeals and Interferences of the
PTO  to  determine,  by  means  of  an  interference   proceeding,   whether  an
interference  should  be  declared  with  issued  U.S.  patents  held by the NIH
relating to oligonucleotide  phosphorothioates.  An interference proceeding is a
proceeding to determine who was the first to invent, and thus who is entitled to
a patent for, a claimed invention.  McDonnell Boehnen Hulbert & Berghoff, a U.S.
patent counsel for Hybridon,  is of the opinion that the UMMC patent application
has a  prima-facie  case for  priority  against  the NIH for an  invention  that
includes phosphorothioate-modified  oligonucleotides.  There can, however, be no
assurance  that the PTO will declare an  interference,  or if it does,  what the
outcome will be. If Hybridon  were to lose the  interference,  its  nonexclusive
license from the NIH of the NIH phosphorothioate  patents would not be affected.
If  Hybridon  were to win the  interference,  others  making,  using or  selling
certain  phosphothioate-modified  oligonucleotides would be required to obtain a
license from Hybridon.

            The PTO also  declared a four-way  interference  involving  two UMMC
U.S.  patents,  for which  Hybridon  is the  exclusive  licensee,  relating to a
particular  type  of  modified  oligonucleotides.  The  other  parties  to  this
interference were Integrated DNA Technologies,  or "IDT," Isis  Pharmaceuticals,
Inc. and Gilead Sciences,  Inc. This  interference was settled in early 1999. In
connection with the settlement,  Hybridon has obtained a nonexclusive license to
certain patents and patent applications owned by IDT that broadly claim chemical
modifications  to  oligonucleotides.  Hybridon has also  granted a  nonexclusive
license to IDT to make,  use, and sell limited  quantities  of  oligonucleotides
incorporating certain of Hybridon's advanced chemistries.

            Under its  licenses,  Hybridon is obligated to pay  royalties on its
net sales of products or processes  covered by the licensed  technology  and, in
some cases,  to pay a percentage  of sublicense  income that it receives.  These
licenses impose various  commercialization,  sublicensing,  insurance, and other
obligations on Hybridon.  If Hybridon  fails to comply with these  requirements,
the license could be terminated.

            Legal  standards  relating  to  the  validity  of  patents  covering
pharmaceutical  and  biotechnological  inventions  and the scope of claims  made
under such  patents are still  developing.  As a result,  Hybridon's  ability to
obtain and enforce  patents  that  protect its drugs is  uncertain  and involves
complex legal and factual questions.

             That   Hybridon   owns  or  licenses   pending  or  future   patent
applications  does not mean  that  patents  based  on  those  applications  will
ultimately be issued. First, to obtain a patent on an invention, one must be the
first to  invent  it or the first to file a patent  application  for it.  Patent
applications in the United States are maintained in secrecy until patents issue,
and  publication of any given  discovery in the scientific or patent  literature
tends  to  lag  behind  actual  date  of  that  discovery  by  several   months.
Consequently,  Hybridon  cannot be certain that the inventors of subject  matter
covered by patents and patent  applications  that it owns or  licenses  were the
first to invent, or the first to file patent applications for, those inventions.

            Others,  including Hybridon's competitors,  also hold issued patents
and patent applications  relating to antisense  technology or particular genetic
targets, including an issued patent in Europe covering the gene MDM2. Holders of
any of these patents or patent  applications  may be able to require Hybridon to
change or cease  making or using  certain  products or  processes,  or obtain an
exclusive or  nonexclusive  license in return for licensing  fees,  which may be
substantial.  In this regard, Hybridon is currently in license negotiations with
the holder of the European  MDM2 patent.  Hybridon may not be able to obtain any
such  licenses at a  reasonable  cost.  Furthermore,  such  licenses may be made
available to  competitors  of Hybridon on an exclusive  or  nonexclusive  basis.
Failure  to  obtain  such  licenses  could  have a  material  adverse  effect on
Hybridon.


                                       21



<PAGE>

Previously, a competitor was granted another European patent relating to certain
types of stabilized synthetic oligonucleotides for use as therapeutic agents for
selectively  blocking the translation of a messenger RNA into a targeted protein
by binding with a portion of the messenger RNA to which the stabilized synthetic
oligonucleotide is substantially complementary. This European patent was revoked
in its entirety in an opposition proceeding before the European Patent Office in
September  1995. The holder of this patent  appealed this decision.  This appeal
was dismissed on February 18, 1999.

            Hybridon  requires its employees,  consultants,  outside  scientific
collaborators,   sponsored   researchers,   and  other   advisors   to   execute
confidentiality  agreements.  These  agreements  provide  that all  confidential
information  developed or made known by Hybridon to the individual is to be kept
confidential,  subject to specific  exceptions.  In the case of  employees,  the
agreements  provide  that all  inventions  conceived by the  individual  are the
exclusive  property of Hybridon.  These  agreements  may not,  however,  provide
meaningful  protection for Hybridon's trade secrets or adequate  remedies in the
event of breach.

            Consistent  with  pharmaceutical  industry and  academic  standards,
Hybridon's   agreements  with  academic  and  research   institutions  and  U.S.
government  agencies may provide that the results of a given  collaboration,  or
any developments that derive from the  collaboration,  will be freely published,
that  information  or  materials  supplied  by  Hybridon  will not be treated as
confidential  and that Hybridon  must  negotiate a license to  developments  and
results in order to commercialize  products  incorporating them. There can be no
assurance that Hybridon will be able  successfully to obtain any such license at
a  reasonable  cost or that  such  developments  and  results  will  not be made
available to competitors of Hybridon on an exclusive or nonexclusive  basis. See
"Business--Academic and Research Collaborations."

GOVERNMENT REGULATION

            Hybridon's research,  clinical development and production activities
are regulated  for safety,  effectiveness  and quality by numerous  governmental
authorities in the United States and other countries.  Hybridon believes that it
is in material compliance with all applicable  federal,  state and foreign legal
and regulatory requirements.

            FDA  Approvals.  In  addition  to product  approvals  by the FDA, as
described  above, the FDA may require that it inspect  Hybridon's  manufacturing
facilities for compliance  with GMP and other  applicable  rules and regulations
before it will permit a product  manufactured  by Hybridon to be marketed in the
United States.  Any material change by Hybridon in its manufacturing  process or
equipment, including relocation of the manufacturing facility, would necessitate
additional FDA review and approval.

            Other  Regulation.  In addition to regulations  enforced by the FDA,
Hybridon also is subject to regulation under the Occupational  Safety and Health
Act and other present and potential future federal,  state or local regulations.
Furthermore,  because Hybridon uses hazardous materials, chemicals, viruses, and
various  radioactive   compounds,   it  must  comply  with  U.S.  Department  of
Transportation  and  Environmental   Protection  Agency  regulations  and  other
federal,  state,  and foreign laws and  regulations  regarding  hazardous  waste
disposal, air emissions,  and waste-water discharge.  Although Hybridon believes
that it complies with these laws and regulations, it cannot completely eliminate
the risk of accidental contamination or injury from these materials.

COMPETITION

            There are a number of companies,  both  privately and publicly held,
that are conducting  research and  development  activities on  technologies  and
products aimed at therapeutic regulation of gene expression, including antisense
drugs.  One competitor of Hybridon has recently  received FDA approval to market
an antisense  therapeutic  product for the treatment of CMV retinitis.  Hybridon
believes that the interest in these technologies and products will increase.  It
is possible that Hybridon's competitors will succeed in developing products that
are more effective than Hybridon's.  Furthermore, Hybridon's proposed drugs will
be competing

                                       22



<PAGE>

with other kinds of drugs. Given the fundamental  differences  between antisense
technology and other drug technologies, antisense drugs may be less effective at
treating some diseases than other kinds of drugs.

            Biotechnology and related pharmaceutical technologies have undergone
and continue to be subject to rapid and  significant  change.  Hybridon  expects
that the  technologies  associated with  biotechnology  research and development
will continue to develop rapidly. Hybridon's future will depend in large part on
its ability to compete with these technologies

            Hybridon has many competitors,  including major  pharmaceutical  and
chemical  companies,  biotechnology  firms,  and universities and other research
institutions.  Many of these competitors have  substantially  greater financial,
technical,  and human  resources  than  Hybridon,  and many  have  significantly
greater experience than Hybridon in undertaking preclinical studies and clinical
trials of new  pharmaceutical  products and obtaining  FDA and other  regulatory
approvals.   Accordingly,   Hybridon's  competitors  may  succeed  in  obtaining
regulatory  approvals for products more rapidly than Hybridon.  Furthermore,  if
Hybridon  receives  approval to commence  commercial sales of products,  it will
also be  competing  with  respect  to  manufacturing  efficiency  and  marketing
capabilities, areas in which it has limited experience.

            HSP also faces  competition,  as  Hybridon's  customers may begin to
produce oligonucelotides  internally or may find other sources.  Hybridon may be
forced to reduce the cost of its products to meet the competition.

EMPLOYEES

            As of June 15, 1999, Hybridon employed 50 individuals full-time,  of
whom 19 held  advanced  degrees.  Eighteen  of these  employees  are  engaged in
research and development activities and eight are employed in finance, corporate
development, and legal and general administrative activities. In addition, 24 of
these employees are employees of HSP, of whom 5 are employed in quality control.
Many  of  Hybridon's  management  and  professional  employees  have  had  prior
experience with  pharmaceutical,  biotechnology,  or medical products companies.
None of Hybridon's  employees is covered by a collective  bargaining  agreement,
and management considers relations with its employees to be good.


                                   PROPERTIES

            Hybridon   leases  its  36,000  square  foot  facility  in  Milford,
Massachusetts,  under a lease that  expires in 2004.  Hybridon  has an option to
extend this lease for two additional five-year terms.

            In addition,  Hybridon  leases  approximately  26,000 square feet of
supplemental  laboratory  space in Cambridge,  Massachusetts  under a lease that
expires April 30, 2007. The annual rent for this space is approximately  $23 per
square foot. Hybridon is currently  subleasing  approximately 20,000 square feet
of this to a third party under a sublease that expires September 30, 2000.

                                LEGAL PROCEEDINGS

            Hybridon is not a party to any  litigation  that it  believes  could
have a material adverse effect on Hybridon or its business.

MARKET FOR REGISTRANT'S COMMON EQUITY AND RELATED STOCKHOLDER MATTERS

            From  January 24, 1996 until  December  2, 1997,  Hybridon's  Common
Stock was traded on the Nasdaq National Market under the symbol "HYBN." Prior to
January 24, 1996, there was no established  public trading market for Hybridon's
Common Stock.


                                       23



<PAGE>

            On December 2, 1997,  Hybridon's  Common Stock was delisted from the
Nasdaq  National  Market and began being quoted on the NASD OTC Bulletin  Board.
Prices reflected on the NASD OTC Bulletin Board may reflect inter-dealer prices,
without  retail  mark-up,  mark-downs  or  commissions  and may not  necessarily
represent actual transactions.

            On December 10, 1997 Hybridon effected a one-for-five  reverse stock
split of its Common  Stock.  As a result of the reverse  stock split,  each five
shares of  Common  Stock was  automatically  converted  into one share of Common
Stock, with cash paid in lieu of any fractional shares.

            The  following  table sets forth for the periods  indicated the high
and low sales  prices per share of the Common  Stock during each of the quarters
set forth  below as  reported  on the  Nasdaq  National  Market and the NASD OTC
Bulletin  Board since  January 24, 1996 and as adjusted to reflect the  December
1997 reverse stock split.
<TABLE>
<CAPTION>

                                                                       HIGH                  LOW
                                                                       ----                  ---
1996

<S>                                                                    <C>                   <C>
First Quarter (from January 24, 1996)...........................      $71.250               $43.750
Second Quarter..................................................       59.375                25.625
Third Quarter...................................................       59.375                33.125
Fourth Quarter..................................................       43.125                26.250


1997

First Quarter...................................................      $43.125               $28.125
Second Quarter..................................................       35.625                25.000
Third Quarter...................................................       28.125                 7.500
Fourth Quarter..................................................        4.859                 2.609

1998

First Quarter...................................................        3.359                 1.000
Second Quarter..................................................         2.75                 1.609
Third Quarter...................................................        2.516                 1.125
Fourth Quarter..................................................         3.25                 1.125

1999

First Quarter...................................................        1.953                 1.000
</TABLE>

            The  reported  closing bid price of the Common Stock on the NASD OTC
Bulletin Board on June 21, 1999 was $0.4375 per share.


                                 DIVIDEND POLICY

            The  Convertible  Preferred  Stock pays dividends at 6.5% per annum,
payable semi-annually in arrears.  These dividends may be paid either in cash or
in  additional  shares of  Convertible  Preferred  Stock,  at the  discretion of
Hybridon.

            Hybridon  has never  declared or paid cash  dividends on its capital
stock,  and Hybridon does not expect to pay any dividends on its Common Stock or
any cash dividends on the Convertible Preferred Stock in the foreseeable future.
The Indenture under which Hybridon issued the 9% Notes on April 2, 1997,  limits
Hybridon's  ability to pay dividends or make other  distributions  on its Common
Stock or to pay cash dividends


                                       24



<PAGE>

on the  Convertible  Preferred  Stock.  As of March 31,  1999,  $1.3  million in
aggregate principal amount of the 9% Notes remained outstanding.

            In  addition,  Hybridon  is  currently  prohibited  from paying cash
dividends under the loan held by the Lender.  See  "Management's  Discussion and
Analysis of  Financial  Condition  and Results of  Operation  -- 1998  Financing
Activities -- Credit Facility."


                                 USE OF PROCEEDS

            Hybridon  will  not  receive  any  proceeds  from  the  sale  of the
Securities by the Selling  Security Holders other than proceeds upon exercise of
certain Hybridon  warrants.  Those proceeds will be added to Hybridon's  general
working capital.


                                 CAPITALIZATION

            The  following  table  sets  forth as of March 31,  1999 the  actual
capitalization  of Hybridon.  See Hybridon's  unaudited  Consolidated  Financial
Statements  as of  March  31,  1999,  included  elsewhere  in this  Registration
Statement (in thousands, except share data.)
<TABLE>
<CAPTION>

<S>                                                                                                      <C>
Current portion of long-term debt ........................................................              $6,073
                                                                                                        ======
Long-term debt, net of current portion ...................................................               $ 454
9% Convertible Subordinated Notes due 2004 ...............................................               1,306
Stockholders' Deficit:
       Preferred Stock, $.01 par value, 5,000,000 shares authorized;
                no shares issued and outstanding .........................................              ------

       Series A Convertible Preferred Stock, $.01 par value, 1,500,000
       shares designated; 641,259 shares issued and outstanding...........................                   6

       Common Stock, $.001 par value, 100,000,000 shares authorized;
       15,304,825 shares issued and outstanding (1).......................................                  15

       Additional Paid-In-Capital ........................................................             242,674
       Accumulated deficit ...............................................................            (242,456)
       Deferred compensation  ............................................................                (895)
                                                                                                        ------
                       Total stockholders' deficit .......................................                (656)
                                                                                                        ------
                               Total Capitalization ......................................              $1,104
                                                                                                        ======
</TABLE>

- -------------------------

(1)    Excludes an aggregate of 14,213,378  shares of Common Stock issuable upon
       exercise of options and warrants  outstanding  as of March 31, 1999, at a
       weighted average exercise price of $6.48 per share.


                                       25



<PAGE>

                             SELECTED FINANCIAL DATA

            The selected  consolidated balance sheet data set forth below, as of
December 31, 1997 and 1998, and the  consolidated  statements of operations data
for each of the three years in the period  December 31,  1998,  are derived from
Hybridon's  Consolidated  Financial Statements which have been audited by Arthur
Andersen LLP,  independent public accountants,  and which are included elsewhere
in this Prospectus.  The selected consolidated financial data as of December 31,
1994,  1995 and 1996 and for the  years  ended  December  31,  1994 and 1995 are
derived from Hybridon's  Consolidated  Financial Statements not included in this
Prospectus,  all of which have been audited by Arthur Andersen LLP,  independent
public accountants. The selected financial data as of March 31, 1999 and for the
three months ended March 31, 1998 and 1999 are derived from Hybridon's unaudited
Consolidated   Financial   Statements  which  are  included  elsewhere  in  this
Prospectus  and which  include,  in the  opinion of  Hybridon,  all  adjustments
(consisting only of normal recurring  adjustments) that are necessary for a fair
presentation  of its financial  position and the results of its  operations  for
those periods.  Operating  results for the three months ended March 31, 1999 are
not  necessarily  indicative  of the results that may be expected for the fiscal
year ending December 31, 1999. The selected  consolidated  financial data should
be read in  conjunction  with,  and are qualified by reference to  "Management's
Discussion  and  Analysis of  Financial  Condition  and Results of  Operations,"
Hybridon's Consolidated Financial Statements and notes thereto and the Report of
Independence Public Accountants included elsewhere in this Prospectus.

<TABLE>
<CAPTION>

                                                                                                                      Three Months
                                                                         Years Ended December 31,                   Ended March 31,

                                                       ----------------------------------------------------------------------------
                                                           1994        1995        1996       1997       1998          1998    1999
                                                           ----        ----        ----       ----       ----          ----    ----
                                                                         (In Thousands, except per share data)          (Unaudited)
Statement of Operations Data:
Revenues
<S>                                                      <C>         <C>         <C>            <C>      <C>        <C>        <C>
       Research and development......................   $ 1,032     $ 1,186     $ 1,419        $945     $1,100     $ 150      $150
       Product and service revenue...................        --          --       1,080       1,877      3,254       825     1,530
       Royalty income................................        --          --          62          48          -        --        40
       Interest income...............................       135         219       1,447       1,079        148        18        53
                                                       --------     -------    --------    --------    -------   -------   -------
                                                          1,167       1,405       4,008       3,949      4,502       993     1,773
Operating Expenses
       Research and development......................    20,024      29,685      39,390      46,828     20,977     6,403     3,447
       General and administrative....................     6,678       6,094      11,347      11,026      6,573     1,665     1,122
       Interest......................................        69         173         124       4,536      2,932     1,607       170
       Restructuring.................................        --          --          --      11,020         --        --        --
                                                       --------     -------    --------    ========    =======   -------   -------

                    Total operating expenses.........    26,771      35,952      50,861      73,410     30,482     9,675     4,739
                                                         ------     -------      ------    ========    =======   -------   -------
Loss from operations.................................   (25,604)    (34,547)    (46,853)    (69,461)   (25,980)   (8,682)   (2,966)
Extraordinary item:
       Gain on conversion of 9% convertible
       subordinated notes payable....................        --          --          --          --      8,877        --        --
                                                       --------    --------    --------    ========   ========   -------   -------
Net loss.............................................   (25,604)    (34,547)    (46,853)    (69,461)   (17,103)   (8,682)   (2.966)
Accretion of preferred stock dividend................        --          --          --          --      2,689        --     1,042
                                                       --------    --------    --------    ========   ========   -------   -------
Net loss to common stockholders......................  $(25,604)   $(34,547)   $(46,853)   $(69,461)  $(19,792)  $(8,682)  $(4,008)
                                                       ========    ========    ========    =========  =========  =======   =======

Basic and diluted net loss per per common share from:
       Operations.................................... $  (70.77)    $(94.70)    $(10.24)    $(13.76)    $(2.19)  $ (1.72)  $ (0.19)
       Extraordinary gain............................        --          --          --          --       0.75        --        --
                                                       --------    --------    --------   =========   ========   -------   -------
                    Net loss per share...............    (70.77)     (94.70)     (10.24)     (13.76)     (1.44)    (1.72)    (0.19)
Accretion of preferred stock dividends...............        --          --          --          --     (0.23)        --     (0.07)
                                                       --------    --------     -------   =========   ========   -------   -------
Net loss per share applicable to common stockholders.   $(70.77)    $(94.70)    $(10.24)    $(13.76)   $ (1.67)  $ (1.72)   $(0.26)
                                                       ========    ========     =======   =========   ========   =======   =======
Shares Used in Computing Basic and
  Diluted Net Loss per Common Share(1)...............       362         365       4,576       5,050     11,859     5,060    15,305
                                                       ========    ========     =======   =========   =========  ========  =======
</TABLE>

                                       26



<PAGE>

<TABLE>
<CAPTION>

Balance Sheet Data:
                                                                                 December 31,                           March 31,
                                                                                                                          1999
                                                             1994        1995        1996        1997       1998       (unaudited)
                                                          ------------------------------------------------------------------------

<S>                                                         <C>         <C>         <C>           <C>       <C>          <C>
Cash, cash equivalents and short-term investments(2).....  $  3,396    $  5,284    $ 16,419      $2,202    $ 5,607      $   2,461
Working capital (deficit) ...............................    (1,713)        210       8,888     (24,100)    (5,614)        (7,820)
Total assets.............................................    11,989      19,618      41,537      35,072     16,536         12,769
Long-term debt and capital lease
       obligations, net of current portion...............     1,522       1,145       9,032       3,282        473            454
9% Convertible Subordinated
Notes Payable ...........................................        --          --          --      50,000      1,306          1,306
Accumulated deficit .....................................
                                                            (67,794)   (102,341)   (149,194)   (218,655)  (238,448)      (242,456)
Total stockholders' equity (deficit).....................     4,774      12,447      22,855     (46,048)     2,249           (656)
                                                           --------    --------     -------    --------   --------       --------
</TABLE>


(1)   Computed  on the  basis  described  in Notes  2(B)  and  19(C) of Notes to
      Consolidated Financial Statements attached as APPENDIX A hereto.

(2)   Short-term  investments  consisted  of  U.S.  government  securities  with
      maturities  greater  than  three  months  but less  than one year from the
      purchase date.


                                       27



<PAGE>

           MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION
                            AND RESULTS OF OPERATIONS

GENERAL

            Hybridon  is engaged in the  discovery  and  development  of genetic
medicines  based on  antisense  technology.  Hybridon  commenced  operations  in
February  1990 and since that time has been  engaged  primarily  in research and
development  efforts,  developing its  manufacturing  capabilities,  and raising
capital. In order to commercialize its therapeutic products,  Hybridon will need
to address a number of  technological  challenges and comply with  comprehensive
regulatory  requirements.  All  revenues  received by Hybridon to date have been
derived from collaborative  agreements,  interest on invested funds and revenues
from the custom contract  manufacturing of synthetic DNA and reagent products by
Hybridon Specialty Products ("HSP").

            Hybridon has incurred cumulative losses from inception through March
31, 1999 of approximately  $242.5 million.  Hybridon implemented a restructuring
plan in the second half of 1997, which has  significantly  reduced its operating
expenses.  However,  Hybridon expects that its research and development expenses
will be  significant  in 1999 and  future  years  as it  pursues  its core  drug
development  programs and expects to continue to incur operating losses and have
significant  capital  requirements  that it will  not be  able to  satisfy  with
internally  generated  funds.  As of June 15, 1999, the Company had 50 full-time
employees.

RESTRUCTURING PLAN

            During the second half of 1997, Hybridon implemented a restructuring
plan to reduce  expenditures on a phased basis in an effort to conserve its cash
resources.  As part of this plan, in addition to terminating  the development of
GEM(R) 91, Hybridon reduced or suspended programs unrelated to its core advanced
chemistry antisense drug development  programs.  In addition,  in 1997, Hybridon
terminated the employment of a substantial  number of employees at its Cambridge
and  Milford,  Massachusetts  and Paris,  France  facilities  and  substantially
reduced operations at its Paris, France office. In December 1998, Hybridon began
the final process of terminating all operations in Europe.

            In 1997  Hybridon  subleased a portion of each of its  facilities in
Cambridge,   Massachusetts  (including  a  substantial  portion  of  its  former
headquarters). In June 1998, Hybridon relocated its headquarters from Cambridge,
Massachusetts to its facility in Milford,  Massachusetts  and subsequently  sold
its interest in Charles River Building  Limited  Partnership,  or the "Cambridge
Landlord,"  which owned the former  Cambridge  headquarters.  In connection with
this  transaction and the  termination of the Cambridge lease in 1998,  Hybridon
received  $6,163,000  in cash,  which  included  the  return of a portion of its
security  deposit for its Cambridge  headquarters  and the  reclassification  on
Hybridon's  balance  sheet of  $660,000  from  restricted  cash to cash and cash
equivalents.  The Cambridge  facility was re-leased in September 1998 to a third
party, subject to a sublease to a portion of the premises.  As a result of these
actions,  Hybridon was relieved of its  substantial  lease  obligations  for the
Cambridge facility, subject to a continuing liability for any defaults which may
arise under the sublease.

RESULTS OF OPERATIONS

Three Months Ended March 31, 1999 and 1998

            Hybridon  had total  revenues of $1.8  million for the three  months
ended March 31,  1999,  compared  with $1.0 million for the same period in 1998.
Product and service revenue increased to $1.5 million for the three months ended
March 31,  1999,  compared  with $0.8  million for the same period in 1998.  The
increase was  primarily  the result of an  expansion  of the HSP customer  base,
increased sales to certain existing  customers,  and the receipt of $0.1 million
in service revenue from MethylGene  pursuant to an agreement entered into in May
1998.


                                       28



<PAGE>

            Revenues  from  interest  income  increased  to $0.1 million for the
three months ended March 31, 1999,  from a nominal amount for the same period in
1998. The change was primarily the result of higher cash balances  available for
investment during the quarter.

            Hybridon's  research  and  development  expenses  decreased  to $3.4
million for the three  months  ended March 31,  1999,  from $6.4 million for the
same  period  in 1998.  The  decrease  reflects  Hybridon's  restructuring  that
commenced  during  the  second  half  of  1997  and  continued  into  1998.  The
restructuring   included  the   discontinuation   of  operations  at  Hybridon's
facilities in Europe,  termination  of the clinical  development of GEM(R)91 and
the reduction or suspension of selected  programs  unrelated to Hybridon's  core
advanced  chemistry  antisense  drug  development   program.  The  restructuring
resulted in significant  reductions in employee-related  expenses,  clinical and
outside testing, consulting,  materials and lab expenses. Accordingly,  research
and  development  salaries  and  related  costs  decreased  in  1999  due to the
reduction in the number of employees engaged in research and development.

            The facilities expense included in research and development expenses
decreased  significantly  in 1999 as a result of the relocation of the Company's
corporate  offices  in July  1998  from  Cambridge  to  Milford,  Massachusetts.
Hybridon's  facility costs in 1999 related to research and development were also
reduced by the income  received  from  subleasing  its  underutilized  Cambridge
facilities before the relocation.

            Hybridon's  general and  administrative  expenses  decreased to $1.1
million for the three  months  ended March 31,  1999,  from $1.7 million for the
same period in 1998.  The decrease  reflects  Hybridon's  restructuring  program
initiated during the second half of 1997 and its effect on employee-related  and
consulting  expenses and net facilities costs and increased legal and accounting
fees resulting from the Hybridon's financing activities in 1998.

            The  facilities  expense  included  in  general  and  administrative
expenses also decreased  significantly  in 1999 as a result of the relocation of
the Company's  corporate  offices to Milford,  Massachusetts.  Facility costs in
1999 were also  reduced by the income  received  from  subleasing  underutilized
Cambridge facilities before the relocation.  General and administrative expenses
related  to  business  development,  public  relations  and legal  expenses  all
decreased in 1999.

            Hybridon's  patent expenses remained at approximately the same level
in 1999, as Hybridon  continued to limit the scope of patent  protection that it
sought as part of its effort to conserve its cash resources,  while  prosecuting
and maintaining key patents and patent applications.

            Hybridon's  interest expense decreased to $0.2 million for the three
months ended March 31, 1999,  from $1.6 million for the same period in 1998. The
decrease is attributable to the exchange of  approximately  $48.7 million of the
9%  convertible  subordinated  notes  (the " 9%  Notes"),  issued in the  second
quarter of 1997, for Series A Preferred  Stock on May 5, 1998. In addition,  the
outstanding  balance of  borrowings  to finance the  purchase  of  property  and
equipment was reduced in May 1998, resulting in a reduction in interest expense.

            As a result of the above  factors,  Hybridon  incurred a net loss of
$3.0 million for the three months ended March 31, 1999, compared with a net loss
of $8.7  million  for the three  months  ended  March  31,  1998.  Hybridon  had
accretion  of  preferred  stock  dividends  of $1.0 million at March 31, 1999 to
reflect  the 1999  portion  of  dividends  payable  to the  holders  of Series A
Preferred  Convertible Stock,  resulting in a net loss to common stockholders of
$4.0 million for the three months ended March 31, 1999.

Years Ended December 31, 1996, 1997 and 1998

            Hybridon had total revenues of $4.0 million in 1996, $3.9 million in
1997, and $4.5 million in 1998.  During 1996, 1997 and 1998,  Hybridon  received
revenues  from research and  development  collaborations  of $1.4 million,  $0.9
million and $1.1 million,  respectively.  Research and development collaboration
revenues decreased in 1997 from 1996 because of the cancellation by Roche of its
collaboration with Hybridon and the resulting elimination of research funding by
Roche.  Research and development  collaboration  revenues increased in 1998 from
1997,  primarily due to Hybridon  receiving  certain  payments under its license
agreement with MethylGene, Inc.


                                       29



<PAGE>

            Product and service revenues were $1.1 million in 1996, $1.9 million
in 1997 and $3.3 million in 1998. The increase in revenues in 1997 over those in
1996 resulted from a full year of operations for HSP, which commenced operations
in the third quarter of 1996. As of December 31, 1998, HSP had a backlog of $0.9
million.  Hybridon  anticipates  filling this backlog in the first half of 1999.
The increase in revenues in 1998 was primarily the result of an expansion by HSP
in the customer base and increased sales to certain existing customers,  and was
also due in part to Hybridon  receiving  $0.4  million in service  revenue  from
MethylGene.

            Revenues  from  interest  income  were $1.4  million  in 1996,  $1.1
million in 1997 and $0.1  million in 1998.  The  decrease in interest  income in
1997 from  1996,  and in 1998 from 1997 was the  result of lower  cash  balances
available for investment each year.

            During 1996, 1997 and 1998,  Hybridon expended $39.4 million,  $46.8
million and $21.0 million, respectively, on research and development activities.

            The increases in research and development expenses in 1997 from 1996
reflected  increasing  expenses related  primarily to ongoing clinical trials of
Hybridon's  product  candidates,  including (a) clinical trials of two different
formulations of GEM(R) 132, which were first initiated  during the third quarter
of 1996 and the first quarter of 1997,  (b) clinical  trials of GEM(R) 92, which
were  initiated in the third  quarter of 1997 and (c) clinical  trials of GEM(R)
91, which were  initiated in France in October 1993 and in the U.S. in May 1994,
and were  terminated  in July  1997.  Clinical  expenses  related  to  GEM(R) 91
decreased significantly during the second half of 1997 after Hybridon terminated
development of this compound.  Research and development  expenses also increased
in 1997 over 1996 due to significant  increases in preclinical expenses incurred
to meet the  filing  requirements  to  initiate  clinical  trials of  Hybridon's
product candidates in the United States.

            The decrease in research and  development  expenses in 1998 reflects
Hybridon's  restructuring  that  commenced  during the second half of 1997.  The
restructuring   included  the   discontinuation   of  operations  at  Hybridon's
facilities in Europe,  termination of the clinical  development of GEM(R) 91 and
the reduction or suspension of selected  programs  unrelated to Hybridon's  core
advanced  chemistry  antisense  drug  development   program.  The  restructuring
resulted in significant  reductions in employee-related  expenses,  clinical and
outside testing, consulting, materials and lab expenses.

            The facilities  expense related to the research and development area
increased  significantly  in 1997 as a result of the relocation of the corporate
offices to Cambridge,  Massachusetts  and decreased  significantly  in 1998 as a
result of the relocation in July 1998 from Cambridge to Milford,  Massachusetts.
Hybridon's  facility costs in 1998 related to research and development were also
reduced by the income  received  from  subleasing  its  underutilized  Cambridge
facilities.

            Research  and  development  salaries and related  costs  remained at
approximately  the same level in 1997 as 1996  because of the costs  involved in
terminating  employees in 1997.  Research and  development  salaries and related
costs decreased in 1998 from 1997 due to the substantial reduction in the number
of employees engaged in research and development in 1998.

            Patent  expenses  also remained at  approximately  the same level in
1998 as 1997 and  1996,  as  Hybridon  continued  to limit  the  scope of patent
protection  that it sought as part of its effort to conserve its cash resources,
while prosecuting and maintaining key patents and patent applications.

            Hybridon  incurred  general  and  administrative  expenses  of $11.3
million in 1996, $11.0 million in 1997 and $6.6 million in 1998.

            The decrease in general and administrative expenses in 1998 resulted
primarily from Hybridon's restructuring program initiated during the second half
of 1997 and its  effect on  employee-related  and  consulting  expenses  and net
facilities costs.

            The  facilities  expense  related to the general and  administrative
area increased  significantly in 1997 over 1996 as a result of the relocation of
the corporate offices to Cambridge, Massachusetts. However, as a result of


                                       30



<PAGE>

the  implementation of the  restructuring  plan in the second half of 1997, such
increase  was offset by  decreases  in general and  administrative  salaries and
related  costs and in  consulting  expenses  in the second  half of 1997,  which
carried over into 1998. Hybridon's facilities expense related to the general and
administrative  area  decreased  significantly  in  1998  as  a  result  of  its
relocation to Milford,  Massachusetts.  Facility costs in 1998 were also reduced
by the income  received  from  subleasing  underutilized  Cambridge  facilities.
General and  administrative  expenses  related to business  development,  public
relations  and legal  expenses  decreased  in 1998 from 1997,  but  remained  at
approximately the same level in 1997 as 1996.

            Interest  expense was $0.1 million in 1996, $4.5 million in 1997 and
$2.9  million  in 1998.  The  decrease  in  interest  expense  in 1998 is mainly
attributable to the exchange of approximately  $48.7 million of the 9% Notes for
Series A Preferred Stock on May 5, 1998. In addition, the outstanding balance of
borrowings  to finance the purchase of property and equipment was reduced in May
1998, resulting in a reduction in interest expense.

            The  increase in  interest  expense in 1997 from 1996  reflected  an
increase in Hybridon's debt  outstanding  associated with the issuance of the 9%
Notes and interest  incurred on  borrowings  to finance the purchase of property
and equipment.

            As a part of its  restructuring  plan,  Hybridon  recorded  an $11.0
million restructuring charge in 1997 to provide for (i) the termination costs of
certain research  programs and other contracts,  (ii) the loss of certain leased
facilities  (net of  sublease  income  and other  contracts),  (iii)  severance,
benefits and related costs for 95  terminated  employees and (iv) the write down
of assets to net realizable value.

            As a result of the  above  factors,  Hybridon  incurred  net  losses
before  extraordinary  items of $46.9 million in 1996, $69.5 million in 1997 and
$26.0 million in 1998. Hybridon had extraordinary income of $8.9 million in 1998
resulting  from the  exchange  of 9% Notes for Series A  Preferred  Stock in the
second  quarter of 1998. In accordance  with  Statement of Financial  Accounting
("SFAS")   No.15,   Accounting  by  Debtors  and  Creditors  for  Troubled  Debt
Restructurings,  Hybridon recorded an extraordinary  gain of approximately  $8.9
million  related  to  the  exchange.   The  extraordinary  gain  represents  the
difference  between the carrying value of the 9% Notes tendered for exchange and
the fair value of the Series A  Preferred  Stock  issued upon the  exchange,  as
determined  by the per share  sales  price of such stock sold in May 1998 in the
private  offering  described  below. As a result of this  transaction,  Hybridon
reduced its net loss before  preferred stock dividends to $17.1 million in 1998.
Hybridon  had an  accretion  of  preferred  stock  dividends  of $2.7 million at
December  31,  1998 to  reflect  the 1998  portion of  dividends  payable to the
holders of Series A  Preferred  Convertible  Stock,  resulting  in a net loss to
common stockholders of $19.8 million for 1998.

LIQUIDITY AND CAPITAL RESOURCES

            During  the  three  months  ended  March  31,  1999,  Hybridon  used
approximately $3.1 million to fund operating activities. The primary use of cash
for  operating  activities  was to fund  Hybridon's  loss  before  accretion  of
preferred  stock  dividends  of $3.0  million.  Hybridon  did not  engage in any
significant  investing  and financing  activities  during the three months ended
March 31, 1999.

            During  the  year  ended  December  31,  1998,   Hybridon   utilized
approximately  $22.7  million to fund  operating  activities  and  approximately
$472,000  for  capital  expenditures.  The  primary  use of cash  for  operating
activities  was to fund  Hybridon's  loss  before  extraordinary  items of $26.0
million.  Capital  expenditures  during 1998 included  amounts  expended for the
build-out  and  equipping  of  Hybridon's  corporate  headquarters  and  primary
research and development  laboratories in its leased  manufacturing  facility in
Milford, Massachusetts.

            Hybridon had cash and cash  equivalents of $2.5 million at March 31,
1999. However,  since that date, Hybridon has expended the majority of such cash
resources and continues to have substantial  obligations to lenders, real estate
landlords,  trade creditors and others. On May 14, 1999, Hybridon's  obligations
included  $1.3 million  principal  amount of 9% Notes,  a $6.0 million loan with
Forum Capital Markets, LLC and others, as described below, $0.5 million of notes
payable and approximately $1.8 million of accounts payable.


                                       31



<PAGE>

Because of Hybridon's financial condition, many trade creditors are only willing
to provide Hybridon with products and services on a cash on delivery basis.

1998 FINANCING ACTIVITIES

            On February 6, 1998,  Hybridon  commenced an offer to the holders of
the 9% Notes to exchange  the 9% Notes for Series A Preferred  Stock and certain
warrants of  Hybridon.  On May 5, 1998,  noteholders  holding  $48.7  million of
principal  and $2.4 million of accrued  interest  tendered  such  principal  and
accrued  interest to Hybridon for 510,505 shares of Series A Preferred Stock and
warrants to purchase  3,002,958 shares of common stock with an exercise price of
$4.25 per share.

            On May 5, 1998,  Hybridon  completed  a private  offering  of equity
securities raising total gross proceeds of approximately  $26.7 million from the
issuance  of  9,597,476  shares  of  common  stock,  114,285  shares of Series A
Preferred  Stock and  warrants to purchase  3,329,486  shares of common stock at
$2.40 per share. The gross proceeds include the conversion of approximately $5.9
million of accounts  payable,  capital lease  obligations and other  obligations
into common stock. Hybridon incurred approximately $1.6 million of cash expenses
related to the private  offering and issued  597,699  shares of common stock and
warrants to purchase  1,720,825 shares of common stock at $2.40 per share to the
placement  agents.  In addition,  Hybridon is  obligated to issue an  additional
300,000 shares in connection with this  transaction.  For more information about
this transaction, see Note 15(c) of the Notes to Consolidated Statements.

Credit Facility

            In December 1996, Hybridon entered into a five-year  $7,500,000 note
payable  with a bank.  The  note  contained  certain  financial  covenants  that
required  Hybridon to maintain minimum tangible net worth and minimum  liquidity
and  prohibited  the  payment  of  dividends.  The note was  payable in 59 equal
installments of $62,500 commencing on February 1, 1997 with a balloon payment of
the then remaining outstanding principal balance due on January 1, 2002. Because
Hybridon was required to make certain  prepayments of principal during 1998, the
outstanding principal balance of the loan at November 16, 1998 was approximately
$2.8  million.  The  lender  granted  Hybridon a waiver of  compliance  with the
minimum  tangible net worth  requirement at December 31, 1998 and March 31, 1999
and the minimum liquidity requirement at April 15, 1999.

            Effective  November 20, 1998, Forum Capital  Markets,  LLC ("Forum")
and certain investors  associated with Pecks Management  Partners Ltd. ("Pecks";
Forum and Pecks  collectively,  the "Lender")  purchased the loan from the bank.
Forum and Pecks are affiliates of two members of Hybridon's  Board of Directors.
In connection with this purchase,  the Lender lent an additional $3.2 million to
Hybridon  so as to  increase  the  outstanding  principal  amount of the note to
$6,000,000. In addition, the terms of the note payable were amended as follows:

o           the maturity was extended to November 30, 2003
o           the interest rate was decreased to 8%
o           interest is payable  monthly in arrears,  with the  principal due in
            full at maturity
o           the note payable is convertible, at the Lender's option, in whole or
            in part,  into shares of common  stock of  Hybridon at a  conversion
            price equal to $2.40 a share
o           the  threshold  of the minimum  liquidity  covenant was reduced from
            $4,000,000 to $2,000,000
o           the note  payable  may not be prepaid,  in whole or in part,  at any
            time prior to December 1, 2000

The other terms of the note payable were unchanged.

For further information about this loan, see Note 7 of the Notes to Consolidated
Financial Statements.

Facility Leases

            As of  December  31,  1998,  Hybridon  has  future  operating  lease
commitments of approximately $7.7 million through 2007 for its existing leases.

                                       32



<PAGE>

Net Operating Loss Carryforwards

            As of December 31, 1998,  Hybridon had approximately  $220.0 million
and  $3.9  million  of  net  operating   loss  and  tax  credit   carryforwards,
respectively.  The Tax  Reform  Act of 1986 (the  "Tax  Act")  contains  certain
provisions that may limit  Hybridon's  ability to utilize net operating loss and
tax credit  carryforwards  in any given year if certain events occur,  including
cumulative  changes in  ownership  interests  in excess of 50% over a three-year
period.  Hybridon has completed  several  financings since the effective date of
the Tax Act, which, as of December 31, 1998, have resulted in ownership  changes
in excess of 50%, as defined  under the Tax Act and which will limit  Hybridon's
ability to utilize its net operating loss carryforwards.

            Since inception,  Hybridon has incurred  significant losses which it
has funded through the issuance of equity securities,  debt issuances,  sales by
HSP,  and  through  research  and  development   collaborations   and  licensing
arrangements.

HISTORY OF OPERATING LOSSES; UNCERTAINTY OF FUTURE PROFITABILITY

            Since inception,  Hybridon has incurred significant losses, which it
has funded through the issuance of equity securities,  debt issuances,  sales by
HSP,  and  through  research  and  development   collaborations   and  licensing
arrangements.

FUTURE CAPITAL NEEDS; UNCERTAINTY OF ADDITIONAL FUNDING

            Hybridon's  ability to continue  operations  in 1999  depends on its
success in obtaining  new funds in the immediate  future.  Hybridon is currently
seeking  debt or  equity  financing  in an  amount  sufficient  to  support  its
operations  through  the  end  of  1999,  and  in  connection  therewith,  is in
negotiations with several parties to obtain such financing.  However,  there can
be no assurance that Hybridon will obtain any funds or as to the timing thereof.
The Company's  existing cash resources and proceeds of accounts  receivable from
HSP customers  are expected to be  sufficient  to fund the Company's  operations
into  July  1999.  The  Company's  management  expects  such  receivables  to be
collected  no later than July 1999,  given such  customers'  payment  histories,
although there can be no assurance  thereof.  If the Company is unable to obtain
substantial  additional  new funding by the end of July,  1999,  Hybridon may be
required  to  further  curtail  significantly  one  or  more  of its  core  drug
development  programs,  obtain funds  through  arrangements  with  collaborative
partners or others that may  require it to  relinquish  rights to certain of its
technologies,  product candidates or products which it would otherwise pursue on
its own or terminate operations or seek relief under applicable bankruptcy laws.

            Even if Hybridon  obtains  sufficient cash to fund its operations in
1999, it will be required to raise substantial additional funds through external
sources,  including  through  collaborative  relationships and public or private
financing,  to support its  operations  beyond  1999.  Except for  research  and
development  funding from Searle under its  collaborative  agreement with Searle
(which is subject to early termination in certain  circumstances),  Hybridon has
no committed  external sources of capital,  and, as discussed above,  expects no
product  revenues for several years from sales of the therapeutic  products that
it is developing (as opposed to sales of DNA products and reagents  manufactured
and sold by HSP).

            No assurance can be given that additional funds will be available to
fund  operations  for the balance of 1999 or in future years,  or, if available,
that such funds will be available on acceptable  terms. If additional  funds are
raised  by  issuing  equity  securities,   further  dilution  to  then  existing
stockholders  will  result.  Additionally,  the  terms  of any  such  additional
financing  may  adversely  affect  the  holdings  or  rights  of  then  existing
stockholders.

            Hybridon's future capital  requirements will depend on many factors,
including  continued  scientific  progress in its research,  drug  discovery and
development programs, the magnitude of these programs, progress with preclinical
and clinical trials,  sales of DNA products and reagents to third parties by HSP
and the  margins  on such  sales,  the  time and  costs  involved  in  obtaining
regulatory  approvals,  the costs involved in filing,  prosecuting and enforcing
patent  claims,  competing  technological  and market  developments,  Hybridon's
ability


                                       33



<PAGE>

to  establish  and maintain  collaborative  academic  and  commercial  research,
development  and  marketing  relationships,  its  ability to obtain  third-party
financing for leasehold  improvements  and other  capital  expenditures  and the
costs  of   manufacturing   scale-up  and   commercialization   activities   and
arrangements.

YEAR 2000; CONTINGENCY PLANS

            As  has  been  widely   publicized,   many   computer   systems  and
microprocessors  are not programmed to  accommodate  dates beyond the year 1999.
Hybridon's  exposure to this year 2000 ("Y2K")  problem  comes not only from its
own internal computer systems and microprocessors, but also from the systems and
microprocessors  of its key suppliers,  including  utility companies and payroll
services.

            Hybridon  believes  that  all of its  internal  systems  will be Y2K
compliant  by the end of the  third  quarter  of  1999.  Hybridon  is  currently
evaluating all of its internal computer systems and  microprocessors in light of
the Y2K problem. As part of this process, Hybridon has conducted an inventory of
its automated  instruments  and other  computerized  equipment and is contacting
applicable vendors for information regarding Y2K compliance.  Hybridon will then
upgrade or otherwise modify its internal  computer systems and  microprocessors,
to the  extent  necessary.  Testing of all its  internal  computer  systems  and
microprocessors  was completed in the first  quarter of 1999.  Hybridon does not
expect the cost of bringing all Hybridon's systems and microprocessors  into Y2K
compliance will be material. Approximately 50% of Hybridon's systems either have
been found compliant or have already been brought into compliance.

            Hybridon's Y2K  compliance  efforts are in addition to other planned
information technology ("IT") projects.  While these efforts have caused and may
continue to cause delays in other IT projects, Hybridon does not expect that any
of these delays will have a significant  effect on  Hybridon's  business or that
any of Hybridon's other IT projects will be canceled or postponed to pay for the
Y2K upgrades.

            With  regard  to  potential  supplier  Y2K  problems,  Hybridon  has
compiled a list of its critical suppliers,  and has sent and received back a Y2K
questionnaire from each of them in order to permit Hybridon to ascertain the Y2K
compliance  status of each.  Hybridon has not yet uncovered any key supplier Y2K
problems that could have a material effect on its business. If through continued
monitoring of these suppliers Hybridon becomes aware of any such problems and is
not satisfied that those problems are being adequately  addressed,  it will take
appropriate steps to find alternative suppliers.

            It has  been  acknowledged  by  governmental  authorities  that  Y2K
problems have the  potential to disrupt  global  economies,  that no business is
immune from the potentially far-reaching effects of Y2K problems, and that it is
difficult to predict with  certainty  what will happen after  December 31, 1999.
Consequently,  it is possible that Y2K problems  will have a material  effect on
Hybridon's  business even if Hybridon takes all  appropriate  measures to ensure
that it and its key suppliers are Y2K compliant.

            It is possible  that the  conclusions  reached by Hybridon  from its
analysis to date will change,  which could cause  Hybridon's  Y2K cost estimates
and target completion dates to change.


                        DIRECTORS AND EXECUTIVE OFFICERS
                                   OF HYBRIDON

EXECUTIVE OFFICERS AND DIRECTORS

            The  following  table sets forth certain  information  regarding the
executive officers and directors of Hybridon as of June 1, 1999.


                                       34



<PAGE>

<TABLE>
<CAPTION>

Name                                                          Age       Position
- ----                                                          ---       --------

<S>                                                           <C>          <C>
E. Andrews Grinstead, III...................................  53        Chairman of Board of Directors (Class III),  President and
                                                                        Chief Executive Officer
Sudhir Agrawal, D. Phil.....................................  45        Senior  Vice  President  of  Discovery,  Chief  Scientific
                                                                        Officer, and Director (Class III)
Nasser Menhall..............................................  42        Director (Class I)
Arthur W. Berry.............................................  56        Director (Class I)
Harold L. Purkey............................................  54        Director (Class I)
James B. Wyngaarden, M.D....................................  73        Director (Class II)
Paul C. Zamecnik, M.D.......................................  85        Director (Class II)
Camille Chebeir.............................................  60        Director (Class II)
Youssef El-Zein.............................................  49        Director (Class III)
H.F. (Jake) Powell..........................................  66        Director (Class III)
</TABLE>


            E.  Andrews  Grinstead,  III  joined  Hybridon  in June 1991 and was
appointed  Chairman of the Board and Chief Executive  Officer in August 1991 and
President in January  1993.  He has served on the Board of Directors  since June
1991. Prior to joining  Hybridon,  Mr. Grinstead served as Managing Director and
Group  Head of the  life  sciences  group  at  Paine  Webber,  Incorporated,  an
investment banking firm, from 1987 to October 1990;  Managing Director and Group
Head of the life sciences group at Drexel Burnham  Lambert,  Inc., an investment
banking firm,  from 1986 to 1987;  and Vice  President at Kidder,  Peabody & Co.
Incorporated,  an investment banking firm, from 1984 to 1986, where he developed
the life sciences  corporate  finance specialty group. Mr. Grinstead served in a
variety of operational  and executive  positions with Eli Lilly and Company,  an
international  pharmaceutical  company,  from  1976 to 1984,  most  recently  as
General  Manager of Venezuelan  Pharmaceutical,  Animal Health and  Agricultural
Chemical Operations and at Eli Lilly Corporate Staff as Administrator, Strategic
Planning and Acquisitions. Since 1991, Mr. Grinstead has served as a director of
Pharmos  Corporation,  a development stage company engaged in the development of
novel  pharmaceutical  compounds and drug delivery  systems.  Mr. Grinstead also
serves as a director of Meridian  Medical  Technologies,  Inc., a pharmaceutical
and medical  device  company.  Mr.  Grinstead was  appointed to The  President's
Council of the  National  Academy of Sciences  and the  Institute of Medicine in
January 1992 and the Board of the  Massachusetts  Biotech Council in 1997. Since
1994,  Mr.  Grinstead  has  served as a member of the Board of  Trustees  of the
Albert B. Sabin Vaccine Foundation, a charitable foundation dedicated to disease
prevention.  Mr. Grinstead received an A.B. from Harvard College in 1967, a J.D.
from the  University  of Virginia  School of Law in 1974 and an M.B.A.  from the
Harvard Graduate School of Business Administration in 1976.

            Sudhir  Agrawal  joined  Hybridon  in  February  1990 and  served as
Principal  Research  Scientist  from  February  1990 to January 1993 and as Vice
President  of  Discovery  from  December  1991 to  January  1993  prior to being
appointed Chief Scientific  Officer in January 1993 and Senior Vice President of
Discovery  in March 1994.  He has served on the Board of  Directors  since March
1993. Prior to joining Hybridon,  Dr. Agrawal served as a Foundation  Scholar at
the  Worcester  Foundation  from 1987  through  1991.  Dr.  Agrawal  served as a
Research  Associate  at Research  Council  Laboratory  of  Molecular  Biology in
Cambridge, England, from 1985 to 1986, studying synthetic oligonucleotides.  Dr.
Agrawal  received a B.Sc. in chemistry,  botany and zoology in 1973, an M.Sc. in
organic  chemistry in 1975 and a D. Phil.  in  chemistry in 1980 from  Allahabad
University in India.

            Nasser  Menhall was  appointed  member of the Board of  Directors of
Hybridon  in 1992.  He has been a member  of the  Board of  Directors  and Chief
Executive Officer of the WorldCare Group, a teleradiology  company,  since 1993;
President of Pillar  Limited,  a private  investment and  management  consulting
firm, since 1990; and President of Biomedical  Associates,  a private investment
firm, since 1990.

            Arthur W. Berry was  appointed  member of the Board of  Directors of
Hybridon in 1998. He has been Chairman and Managing  Partner of Pecks Management
Partners, since 1990, and was Vice President and Co-


                                       35



<PAGE>

Manager  of the  Alliance  Convertible  Securities  Group and  President  of the
Alliance  Convertible Fund from 1985 to 1990. Prior to joining Alliance,  he was
Vice  President  and Head of Special  Funds  Section  and  Manager of the Harris
Convertible  Fund at Harris  Bank and  Senior  Portfolio  Manager  in the bank's
Individual  Investment  Management  Group.  He is also a member  of the Board of
Directors of Intellicorp, Inc.

            Harold L. Purkey was  appointed  member of the Board of Directors of
Hybridon  in  1998.  He is  President  of Forum  Capital  Markets  LLC,  and was
previously  Senior Managing  Director of Convertible  Securities at Smith Barney
Shearson  from 1990 to 1994,  and  Senior  Executive  Vice  President  of Drexel
Burnham Lambert from 1982 to 1989. He is also a member of the Board of Directors
of Richardson Electronics.

            James B.  Wyngaarden was appointed  member of the Board of Directors
of Hybridon in 1990;  he has been Vice  Chairman  of the Board of  Directors  of
Hybridon since February 1997. He was Foreign  Secretary of the National  Academy
of Sciences and the  Institute  of Medicine of the National  Academy of Sciences
from 1990 to 1994; Council member of the Human Genome  Organization from 1990 to
1993 and Director from 1990 to 1991; and Director of the National  Institutes of
Health  from 1982 to 1989.  He is a member of the  Board of  Directors  of Human
Genome Sciences, Inc. and Magainin Pharmaceuticals, Inc.

            Paul C. Zamecnik was  appointed  member of the Board of Directors of
Hybridon in 1990. He was Principal  Scientist at the  Worcester  Foundation  for
Biomedical  Research,  Inc. from 1979 to 1996, and has been Collis P. Huntington
Professor of Oncologic  Medicine  Emeritus at the Harvard  Medical  School since
1979.  He  is  also  currently  Senior  Scientist  and  Honorary   Physician  at
Massachusetts General Hospital in Boston.

            Youssef  El-Zein was  appointed  member of the Board of Directors of
Hybridon  in 1992,  and has been  Vice  Chairman  of the Board of  Directors  of
Hybridon since  February  1997. He has been Executive  Officer of Pillar S.A., a
private investment and management  consulting firm, since 1991;  Chairman of the
WorldCare  Group  since  1993;  and member of the Board of  Directors  of Pillar
Investment  Limited ("Pillar  Investment"),  a private investment and management
consulting firm, since 1991.

            Camille  Chebeir was  appointed  member of the Board of Directors of
Hybridon in 1999.  Since 1995, he has been  President of Sedco  Services,  Inc.,
which manages the worldwide  investments of a prominent Saudi Arabian family. In
that  capacity,  he serves on the  boards of  various  entities  in which  Sedco
Services,  Inc. has invested.  Since 1995, he has served as Managing Director of
MetroWest, a Florida real estate development company. Mr. Chebeir was previously
with National  Commercial  Bank,  and served from 1989 to 1992 as Executive Vice
President   of  its  New  York   branch,   and   from   1983  to  1989  as  Vice
President--Operations.  Mr.  Chebeir is a former  President  of the Arab Bankers
Association of North America.

            Jake  Powell  was  appointed  member  of the Board of  Directors  of
Hybridon in 1999.  From 1982 to 1996,  he was with Nabisco,  Inc.,  serving from
1994 to 1996 as Executive Vice President and Chief Financial  Officer,  and from
1989 to 1994 as  President  of Nabisco  International.  He served as Senior Vice
President and Chief Financial Officer of Standard Brands, Inc. from 1980 to 1981
and held various  executive offices with Standard Brands affiliates from 1974 to
1980.

            Hybridon's  Restated  Certificate  of  Incorporate  provides  for  a
staggered Board of Directors consisting of three classes,  with each class being
as nearly  equal in number as  possible.  At each annual  meeting of  Hybridon's
stockholders,  the term of one class expires and the successors of the directors
in that class are elected for a term of three  years.  Hybridon  has  designated
three Class I directors, three Class II directors, and four Class III directors;
they are  identified  in the above  table.  They are to serve  until the  annual
meeting of  stockholders to be held in 2000,  2001 and 2002,  respectively,  and
until their respective successors are duly elected and qualified, or until their
earlier resignation or removal. The Restated Certificate of Incorporate provides
that directors may be removed only for cause by a majority of stockholders.


                                       36



<PAGE>

                             EXECUTIVE COMPENSATION

COMPENSATION OF EXECUTIVE OFFICERS

Summary Compensation Table

            The following table sets forth the compensation for the fiscal years
ended December 31, 1998 ("fiscal 1998"), December 31, 1997 and December 31, 1996
for the persons,  the Company's  Chief  Executive  Officer and Chief  Scientific
Officer,  who were serving as Executive  Officers at December 31, 1998 and whose
total  annual  salary  and bonus  exceeded  $100,000  in fiscal  1998 (the Chief
Executive  Officer and Chief Scientific  Officer are hereinafter  referred to as
the "Named Executive Officers"):

                           SUMMARY COMPENSATION TABLE
<TABLE>
<CAPTION>

                                                                                                                       LONG-TERM
                                                                                                                     COMPENSATION
                                                                               ANNUAL COMPENSATION                      AWARDS
                                                                               -------------------                   ------------

                                                                                        OTHER
                                                                                        ANNUAL        SECURITIES
                                                                                        COMPEN-       UNDERLYING      ALL OTHER
              NAME AND PRINCIPAL POSITION                    SALARY        BONUS        SATION          OPTIONS      COMPENSATION
              ---------------------------                    ------        -----        ------          -------      ------------

<S>                                                <C>        <C>             <C>       <C>             <C>          <C>
E. Andrews Grinstead, III..........................1998      $375,000        0         $ 93,750(1)     500,000      $ 53,861(2)
   Chairman of the Board,                          1997      $375,000        0         $ 93,750(1)      66,806      $ 53,784(2)
   President and Chief Executive                   1996      $375,000     $225,000     $ 93,750(1)      50,000      $ 48,163(2)
   Officer

Sudhir Agrawal, D. Phil............................1998      $250,000        0         $ 50,000(1)     500,000      $ 22,115(2)
   Senior Vice President of                        1997      $250,000        0         $ 50,000(1)      32,263      $ 13,462(2)
   Discovery, Chief Scientific                     1996      $250,000     $100,000      $ 4,277(1)      25,000      $ 24,399(2)
   Officer and Director
</TABLE>

- ----------------
(1)    Other annual  compensation paid, or to be paid, by the Company to, or for
       the benefit of, the Named Executive Officer is as follows:
<TABLE>
<CAPTION>

E. Andrews Grinstead, III                                            1998                1997             1996
- -------------------------                                            ----                ----             ----

<S>                                                                 <C>                <C>               <C>
Paid in lieu of employee benefits............................       $79,903            $34,902           $76,017

Purchase of life insurance and other
payments to third parties....................................        13,487             58,848            17,733
                                                                    -------            -------           -------

Total........................................................       $93,750            $93,750           $93,750
                                                                    =======            =======           =======


Sudhir Agrawal, D. Phil.                                              1998               1997              1996
- ------------------------                                              ----               ----              ----

Paid in lieu of employee benefits............................       $39,337            $38,132              $ 0

Purchase of life insurance and other payments to
third parties................................................        10,663             11,868            4,277
                                                                    -------            -------           ------

Total........................................................       $50,000            $50,000           $4,277
                                                                    =======            =======           ======
</TABLE>



                                       37



<PAGE>

(2)    All other compensation paid, or to be paid, by the Company to, or for the
       benefit of, the Named Executive Officers is as follows:

<TABLE>
<CAPTION>

E. Andrews Grinstead, III                                        1998            1997             1996
- -------------------------                                        ----            ----             ----

<S>                                                             <C>             <C>              <C>
Surrender of unused vacation days...........................    $37,861         $37,300          $32,163
Additional payments.........................................     16,000          16,484           16,000
                                                                -------         -------          -------

Total.......................................................    $53,861         $53,784          $48,163
                                                                =======         =======          =======


Sudhir Agrawal, D. Phil.                                         1998            1997             1996
- ------------------------                                         ----            ----             ----

Surrender of unused vacation days...........................    $22,115         $13,462          $24,399
                                                                 ------        --------          ------

Total.......................................................   $ 22,115        $ 13,462         $ 24,399
                                                               ========        ========         ========
</TABLE>


Option Grants Table

            The following table sets forth certain information concerning grants
of  stock  options  made  during  fiscal  1998 to each  of the  Named  Executive
Officers:

<TABLE>
<CAPTION>
                        OPTION GRANTS IN LAST FISCAL YEAR

                                INDIVIDUAL GRANTS
                                -----------------


                                                                                                             POTENTIAL REALIZABLE
                                                           PERCENTAGE                                          VALUE AT ASSUMED
                                         NUMBER OF          OF TOTAL                                        ANNUAL RATES OF STOCK
                                         SECURITIES         OPTIONS                                         PRICE APPRECIATION FOR
                                         UNDERLYING         GRANTED TO      EXERCISE                           OPTIONS TERM(2)
                                         OPTIONS          EMPLOYEES IN       PRICE        EXPIRATION     ------------------------
                                         GRANTED           FISCAL YEAR     PER SHARE         DATE(1)        5%             10%
                                         ----------       ------------     ---------       ---------     --------      ----------

<S>                                      <C>                  <C>            <C>            <C>           <C>           <C>
E. Andrews Grinstead, III...........     500,000 (3)          21.4%          $2.00          7/21/08      $323,407      $1,107,416

Sudhir Agrawal, D.Phil..............     500,000 (3)          21.4%          $2.00          7/21/08      $323,407      $1,107,416
</TABLE>

- ---------------

(1)    The expiration date of an option is the tenth  anniversary of the date on
       which the option was originally granted.

(2)    The amounts shown on this table represent  hypothetical  gains that could
       be achieved  for the  respective  options if  exercised at the end of the
       option term. These gains are based on assumed rates of stock appreciation
       of 5% and 10%,  compounded  annually from the date the respective options
       were  granted to their  expiration  date.  The gains shown are net of the
       option exercise price,  but do not include  deductions for taxes or other
       expenses  associated  with the exercise.  Actual gains,  if any, on stock
       option  exercises  will  depend on the future  performance  of the Common
       Stock, the optionholder's continued employment through the option period,
       and the date on which the options are  exercised.  As of April 16,  1999,
       the last sale  price of Common  Stock of the  Company  was lower than the
       exercise price of the options reflected in this table.

(3)    These stock options are currently  exercisable with respect to 350,000 of
       the shares covered  thereby and will become  exercisable  with respect to
       the remaining  shares covered thereby in equal quarterly  installments in
       arrears commencing on July 21, 1999.


                                       38



<PAGE>

Aggregated Option Exercises and Year-End Option Table

            The following  table sets forth certain  information  concerning the
number  and value of  unexercised  options  held by each of the Named  Executive
Officers on December 31, 1998:


                       AGGREGATED OPTION EXERCISES IN LAST
                         FISCAL YEAR AND FISCAL YEAR-END
                                  OPTION VALUES
<TABLE>
<CAPTION>

                                                               NUMBER OF                        VALUE OF
                                                                SHARES                         UNEXERCISED
                                                              UNDERLYING                      IN THE MONEY
                                                               OPTIONS AT                    OPTIONS AT FISCAL
                                                            FISCAL YEAR-END                    YEAR-END(1)
                                                            ---------------                    -----------

                                                              EXERCISABLE/                     EXERCISABLE/
                                                             UNEXERCISABLE                    UNEXERCISABLE
                                                             -------------                    -------------

<S>                                                         <C>       <C>                        <C>
E. Andrews Grinstead, III................................   461,235 / 302,084                    $ -- / --

Sudhir Agrawal...........................................   340,903 / 277,360                  $3,750 / --
</TABLE>

- ---------------------

(1)   The  closing  price for the  Common  Stock as  reported  by The Nasdaq OTC
      Bulletin  Board on December 31, 1998 (the last day of trading in 1998) was
      $1.625.  Value is  calculated on the basis of the  difference  between the
      option  exercise  price and $1.625,  multiplied by the number of shares of
      Common Stock underlying the option.

DIRECTOR COMPENSATION

            Each non-employee director is paid $1,500 for personal or telephonic
attendance at a Board of Directors or committee meeting. Other directors are not
entitled to compensation in their capacities as directors.  All of the directors
are reimbursed for their expenses  incurred in connection with their  attendance
at Board of Director and committee meetings. In addition,  Dr. Zamecnik received
compensation  in the amount of $83,995 in 1998 in connection  with the provision
of certain  consulting  services to the Company.  Of this amount,  Dr.  Zamecnik
received  25,000 shares of Common Stock and warrants to purchase 6,250 shares of
Common Stock in lieu of $50,000 in cash. The remaining $33,995 was paid in cash.
The Company also is a party to consulting,  advisory and other arrangements with
various  directors  and their  affiliates.  For a  description  of the foregoing
arrangements with the Company and certain other transactions between the Company
and affiliates of certain directors, see "Certain Transactions."

            In October 1995, the Company  adopted the 1995 Director Stock Option
Plan (the "Director  Plan").  Under the terms of the Director  Plan,  options to
purchase  1,000  shares of Common  Stock were  granted to each  director  of the
Company, other than Mr. Grinstead and Dr. Agrawal, (a) as of January 24, 1996 at
an exercise  price of $65.625 per share,  (b) as of May 1, 1997,  at an exercise
price of $27.50 per share,  (c) as of May 1, 1998 at an exercise price of $2.375
per share,  and (d) as of May 1, 1999 at an  exercise  price of $1.22 per share.
The Director Plan also provides that options to purchase  5,000 shares of Common
Stock will be granted to each new director  upon his or her initial  election to
the Board of Directors. However, because of the one-for-five reverse stock split
described  below,  options to purchase 1,000 shares of Common Stock were granted
to Messrs.  Chebeir and Powell upon their  appointment to the Board of Directors
in 1999. In addition,  on June 8, 1999,  the Company's  stockholders  approved a
one-time grant of options to purchase 8,000 shares of the Company's Common Stock
at an  exercise  price  of  $0.47  per  share to each  director  other  than Mr.
Grinstead and Dr.  Agrawal.  Annual  options to purchase  5,000 shares of Common
Stock will be  granted  to each  eligible  director  on May 1 of each year.  All
options will vest on the first anniversary of the date of grant (or, in the case
of options  granted  automatically  each year, on April 30 of the year following
the date of the grant);  provided, that the exercisability of these options will
be accelerated upon the occurrence of a change in control


                                       39



<PAGE>

(as defined in the Director Plan). A total of 400,000 shares of Common Stock may
be issued upon the exercise of stock options  granted  under the Director  Plan.
The exercise  price of options  granted  under the Director  Plan will equal the
closing  price of the Common  Stock on the date of grant.  As of June 15,  1999,
options  to  purchase  an  aggregate  of  93,000  shares of  Common  Stock  were
outstanding  under the Director  Plan.  All share  information  set forth in the
Proxy Statement has been adjusted to take into account the one-for-five  reverse
stock split of the Company's Common Stock effected in December 1997.

            Non-employee directors also have received options to purchase Common
Stock of the Company under the Company's  1997 Stock  Incentive  Plan (the "1997
Plan")  and  the  Company's  1995  Stock  Option  Plan  (the  "1995  Plan").  In
particular, in 1998, the Board of Directors voted to grant an option to purchase
50,000  shares of Common  Stock at $2.00  per  share to Dr.  Wyngaarden  and Mr.
El-Zein,  in  recognition  of their  services  as Vice  Chairmen of the Board of
Directors during the previous twelve months. Mr. El-Zein declined this grant. In
addition, in 1998, the Board of Directors voted to grant 50,000 shares of Common
Stock  of the  Company  to  Dr.  Zamecnik  in  recognition  of  his  outstanding
contributions to the Company.

EMPLOYMENT   AGREEMENTS,   TERMINATION  OF  EMPLOYMENT  AND  CHANGE  IN  CONTROL
ARRANGEMENTS

            The Company is party to an employment  agreement with Mr.  Grinstead
for the period  commencing  July 1, 1996 and ending  June 30,  2001.  Under this
agreement,  Mr. Grinstead is currently entitled to receive an annual base salary
of  $375,000.  Mr.  Grinstead  also is eligible to receive (i) a cash bonus each
year related to the attainment of management  objectives  specified by the Board
of Directors and (ii) additional  payments of $16,000 in 1996, 1997 and 1998. In
the event Mr. Grinstead's  employment is terminated by the Company without cause
(as  defined) or by him for good cause (as  defined),  the Company  will pay Mr.
Grinstead  during the 24-month period following his termination a monthly amount
equal to one-twelfth of the sum of Mr.  Grinstead's annual base salary as of the
date of  termination  and the  average  bonus paid to him during the three years
preceding his termination  (the "Average Bonus  Amount").  The Company also will
continue  Mr.  Grinstead's   benefits  for  such  period,   subject  to  earlier
termination under certain circumstances.  If his employment is terminated by the
Company for failure to perform his assigned duties,  he will continue to receive
his annual base salary and benefits during the six-month  period  following such
termination.  Notwithstanding  the foregoing,  in the event that Mr. Grinstead's
employment is terminated for any of the above reasons within 12 months following
a Change in Control (as defined) of the Company,  Mr. Grinstead will be entitled
to receive, in lieu of the payments described above, a lump sum payment equal to
300% of the sum of his annual base salary and his Average Bonus Amount.

            In accordance with the terms of Mr. Grinstead's  previous employment
agreement,  the Company  loaned  $190,000  to Mr.  Grinstead  in  December  1992
pursuant to the terms of a promissory  note bearing simple interest at a rate of
6% per year,  which  originally  provided for the payment of  principal  and all
accrued  interest  on the  earlier of December  23,  1995 or the  expiration  or
termination  of Mr.  Grinstead's  employment  by the  Company,  but is currently
payable on demand.  Such loan remained  outstanding  as of December 31, 1998, at
which date the total unpaid balance of principal and interest was $258,650.

            The Company is party to an employment agreement with Dr. Agrawal for
the  period  commencing  July 1,  1996 and  ending  June 30,  2000.  Under  this
agreement,  Dr.  Agrawal  serves as Senior Vice President of Discovery and Chief
Scientific Officer of the Company and is currently entitled to receive an annual
base salary of  $250,000.  Dr.  Agrawal is eligible to receive a cash bonus each
year related to the attainment of management  objectives  specified by the Chief
Executive  Officer  and the  Board of  Directors.  In the  event  Dr.  Agrawal's
employment is terminated by the Company without cause (as defined) or by him for
good cause (as defined),  the Company will pay Dr.  Agrawal  during the 24-month
period  following his  termination a monthly  amount equal to one-twelfth of the
sum of Dr.  Agrawal's  annual base salary as of the date of termination  and the
average bonus paid to him during the three years preceding his termination  (the
"Average Bonus Amount").  The Company will also continue Dr. Agrawal's  benefits
for such period, subject to earlier termination under certain circumstances.  If
his  employment is terminated by the Company for failure to perform his assigned
duties,  he will continue to receive his annual base salary and benefits  during
the six-month period following such termination.  Notwithstanding the foregoing,
in the event that Dr.  Agrawal's  employment is terminated  for any of the above
reasons  within 12 months  following  a Change in Control  (as  defined)  of the
Company, Dr.


                                       40



<PAGE>

Agrawal will be entitled to receive,  in lieu of the payments described above, a
lump sum  payment  equal to 300% of the sum of his  annual  base  salary and his
Average Bonus Amount.

            The employment  agreements entered into between the Company and each
of Mr. Grinstead and Dr. Agrawal also provide that all stock options held by any
of the Named Executive  Officers  (including  existing options and options to be
granted in the future) shall include terms  providing (i) that in the event that
such Named Executive  Officer's  employment is terminated by the Company without
cause or by him for good cause the  exercisability of such stock options will be
accelerated  by two years  and such  stock  options  will be  exercisable  for a
two-year  period  following  termination  and (ii) that in the event of  certain
changes  in  control  of the  Company,  its  liquidation  or the  sale of all or
substantially  all of its assets,  all such stock  options not then  exercisable
will vest and become  immediately  exercisable.  The  Company is also a party to
registration rights agreements with Mr. Grinstead that provide that in the event
the Company proposes to register any of its securities under the Securities Act,
at any time, with certain exceptions, Mr. Grinstead shall be entitled to include
the  shares of Common  Stock  held by him in such  registration,  subject to the
right of the managing  underwriter of any underwritten  offering to exclude from
such registration for marketing reasons some or all of such shares.  The Company
also is a party to  indemnification  agreements with Mr.  Grinstead  pursuant to
which the Company has agreed to indemnify him for certain liabilities, including
liabilities arising under the Securities Act.

            Stock  options to purchase an aggregate of 207,513  shares of Common
Stock granted to the Named Executive  Officers pursuant to the 1990 Plan provide
that,  upon a change in control  (as  defined  in the 1990  Plan),  all  options
granted thereunder will become fully exercisable.  In addition,  pursuant to the
terms of the employment  agreements entered into between the Company and each of
the Named Executive Officers described above (i) in April 1997, stock options to
purchase an  aggregate of 156,069  shares of Common  Stock  granted to the Named
Executive  Officers  under the Company's  1995 Plan were amended to provide that
such  options  will  become  fully  exercisable  upon a change in control of the
Company,  and (ii) all stock  options  granted to the Named  Executive  Officers
after March 1, 1997 will provide that such options will become fully exercisable
upon a change of control of the Company.

COMPENSATION COMMITTEE INTERLOCKS AND INSIDER PARTICIPATION

            On  June  16,  1998  the  Board  of   Directors   re-established   a
Compensation   Committee  consisting  of  Messrs.  Berry  and  El-Zein  and  Dr.
Wyngaarden.  None of the directors or executive  officers of the Company had any
"interlock"  relationships  to report  during the  Company's  fiscal  year ended
December 31, 1998.

            Since January 1, 1998, the Company has entered into or is engaged in
certain ongoing  transactions  with (i) Pillar S.A., Pillar  Investment,  Pillar
Limited  and  Charles  River  Building  Limited  Partnership,  entities of which
Messrs.  El-Zein and Menhall are affiliates;  (ii) entities advised by Pecks, an
entity of which Mr.  Berry is a principal;  (iii) Forum,  an entity of which Mr.
Purkey is an affiliate;  and (iv) each of Drs.  Wyngaarden  and Zamecnik and Mr.
Powell. See "Certain Transactions."

         SECURITY OWNERSHIP OF CERTAIN BENEFICIAL OWNERS AND MANAGEMENT

            The following  table sets forth certain  information  as of June 15,
1999 with respect to the beneficial  ownership of shares of Common Stock by each
person known to the Company to own beneficially  more than 5% of the outstanding
shares of Common Stock, assuming conversion of all convertible debt or preferred
stock and exercise of all warrants and stock  options by such person and only by
such person.



                                       41



<PAGE>

<TABLE>
<CAPTION>

                                                                       Amount and Nature
                                                                   of Beneficial Ownership(1)
Name and Address of                                             Number of          Percent of
Beneficial Owner                                                 Shares              Class
- ----------------                                                ---------          ----------

5% STOCKHOLDERS

<S>                                                            <C>                    <C>
Pecks Management Partners Ltd..............................   8,441,556(2)            34.87%
One Rockefeller Plaza
New York, New York  10022

Forum Capital Markets LLC..................................   4,923,415(3)            24.75%
53 Forest Ave.
Old Greenwich, CT  06870

General Motors Employees...................................   3,744,926(4)            19.19%
Domestic Group Trust
c/o Pecks Management Partners Ltd.
One Rockefeller Plaza
New York, New York  10020

Guardian Life Insurance....................................   3,171,698(5)            16.75%
Company of America
201 Park Avenue South, 7A
New York, New York  10003

Intercity Holdings Ltd.....................................   2,216,666(6)            13.73%
c/o Cuson Milner House
18 Parliament Street
Hamilton, Bermuda

Abdelah Bin Mahfouz........................................   2,216,666(7)            13.73%
c/o SEDCO
P.O. Box 4384
Jeddah 21491
Saudi Arabia

Delaware State Employees...................................   2,493,698(8)            13.66%
Retirement Fund
c/o Pecks Management Partners Ltd.
One Rockefeller Plaza
New York, New York  10020



                                       42


<PAGE>

Youssef El-Zein............................................   1,748,722(9)            10.25%
28 Avenue de Messine
75008 Paris, France

Nasser Menhall.............................................   1,726,734(10)           10.14%
28 Avenue de Messine
75008 Paris, France

Pillar Investment Limited..................................   1,617,373(11)            9.56%
28 Avenue de Messine
75008 Paris, France

Yahia M. A. Bin Laden......................................   1,373,977(12)            8.59%
2 rue Charles Bonnet
1206 Geneva, Switzerland

Nicris Limited.............................................   1,360,644(13)            8.50%
c/o Magnin Dunand & Associes
2 rue Charles Bonnet
1206 Geneva, Switzerland

Lincoln National Life Insurance Co.........................   1,246,917(14)            7.33%
c/o Lynch & Mayer
520 Madison Avenue
New York, New York  10022

Faisal Finance Switzerland SA..............................   1,043,112(15)            6.52%
84 Ave Louis Casi
1216 Geneva, Switzerland

Finova Technology Finance Inc. ............................     896,875 (16)           5.60%
10 Waterside Drive
Farmington, CT  06032

Declaration of Trust for the...............................     870,100(17)            5.23%
Defined Benefit Plan of ICI
American Holdings, Inc.
c/o Pecks Management Partners Ltd.
One Rockfeller Plaza
New York, New York  10022
</TABLE>

- ---------------------

(1)   The  number  of  shares  beneficially  owned  is  determined  under  rules
      promulgated by the Securities and Exchange  Commission (the "Commission"),
      and the information is not necessarily  indicative of beneficial ownership
      for any other purpose. Under such rules, beneficial ownership includes any
      shares  as to which the  individual  has sole or  shared  voting  power or
      investment power and also any shares which the individual has the right to
      acquire  within 60 days after June 15, 1999  through  the  exercise of any
      stock option or other right. The inclusion herein of such shares, however,
      does not constitute an admission that the named stockholder is a direct or
      indirect beneficial owner of such shares. Unless otherwise indicated, each
      person or entity named in the table has sole voting  power and  investment
      power (or shares  such power with his or her spouse)  with  respect to all
      shares of capital stock listed as owned by such person or entity.

(2)   Includes  255,381 shares of Series A Convertible  Preferred Stock owned by
      six  investment  advisory  clients of Pecks,  which  clients would receive
      dividends  and the proceeds  from the sale of such shares.  Three of these
      clients are Delaware  State  Employees  Retirement  Fund,  General  Motors
      Employees  Domestic  Group Trust and  Declaration of Trust for the Defined
      Benefit  Plan of ICI  American  Holdings,  Inc.  These  shares of Series A
      Convertible  Preferred  Stock are  convertible  into  6,008,965  shares of
      Common  Stock of the Company.  This amount also  includes  762,419  shares
      issuable upon the exercise of Class A warrants and 420,172 shares issuable
      upon  the  exercise  of  Class D  warrants  held in the  aggregate  by the
      foregoing  entities.  This number also includes  1,250,000 shares issuable
      upon


                                       43



<PAGE>

      conversion of a portion of the  $6,000,000  bank loan to the Company owned
      by certain of the foregoing entities.

(3)   Includes (a) 328,677  shares  issuable  upon exercise of Class B warrants,
      (b) 280,517  shares  issuable  upon the exercise of Class C warrants,  (c)
      408,112  shares  issuable upon  exercise of Class A warrants,  (d) 761,568
      shares  issuable upon  exercise of other  warrants,  (e) 1,250,000  shares
      issuable upon conversion of Forum's portion of the $6,000,000 bank loan to
      the Company,  and (f) 1,098,282  shares issuable upon conversion of 46,677
      shares of Series A Convertible Preferred Stock owned by Forum.

(4)   Includes 114,177 shares of Series A Convertible  Preferred Stock which are
      convertible  into  2,686,518  shares of Common Stock of the Company.  This
      amount also includes  492,783 shares issuable upon the exercise of Class A
      warrants and 565,625 shares  issuable upon  conversion of a portion of the
      $6,000,000 bank loan to the Company owned by this entity.

(5)   Includes 109,067 shares of Series A Convertible  Preferred Stock which are
      convertible  into  2,566,282  shares of Common Stock of the Company.  This
      amount also includes  353,316 shares issuable upon the exercise of Class A
      warrants  and  252,100  shares  issuable  upon  the  exercise  of  Class D
      warrants.

(6)   Includes  375,000  shares  issuable  upon the exercise of Class B warrants
      held by Intercity Holdings Ltd.

(7)   Includes  1,841,666  shares held by  Intercity  Holdings  Ltd. and 375,000
      shares  issuable  upon  exercise  of Class B  warrants  held by  Intercity
      Holdings.  Mr. Mahfouz,  a controlling  stockholder of Intercity  Holdings
      Ltd.,  may be  considered  a beneficial  owner of the shares  beneficially
      owned by such entity.

(8)   Includes  73,536 shares of Series A Convertible  Preferred Stock which are
      convertible  into  1,730,259  shares of Common Stock of the Company.  This
      amount also includes  137,918 shares issuable upon the exercise of Class A
      warrants,  270,271  shares  issuable upon the exercise of Class D warrants
      and 355,250 shares  issuable upon  conversion of portion of the $6,000,000
      bank loan to the Company owned by this entity.

(9)   Includes (a) 82,183 shares  issuable upon the exercise of warrants held by
      Mr. El-Zein, (b) 366 shares issuable upon the exercise of warrants held by
      Pillar  Associated,  (c)  20,000  shares  issuable  upon the  exercise  of
      warrants held by Pillar S.A., (d) 20,000 shares issuable upon the exercise
      of warrants held by Pillar  S.A.R.L.,  (e) 37,500 shares issuable upon the
      exercise  of Class C  warrants  held by  Pillar  Investment  Limited,  (f)
      473,598  issuable  upon the exercise of advisory  warrants  held by Pillar
      Investment  Limited,  (g) 638,032  shares  issuable  upon the  exercise of
      placement  warrants held by Pillar  Investment  Limited,  (h) 5,243 shares
      issuable  upon the exercise of other  warrants  held by Pillar  Investment
      Limited,  (i) 462,800 shares held by Pillar  Investment  Limited,  and (j)
      9,000  shares  issuable  upon the  exercise of stock  options  held by Mr.
      El-Zein.  Mr.  El-Zein,  an affiliate of Pillar  Associated,  Pillar S.A.,
      Pillar  S.A.R.L.  and  Pillar  Investment  Limited,  may be  considered  a
      beneficial owner of the shares beneficially owned by such entities.

(10)  Includes (a) 60,195 shares  issuable upon the exercise of warrants held by
      Mr. Menhall, (b) 366 shares issuable upon the exercise of warrants held by
      Pillar  Associated,  (c)  20,000  shares  issuable  upon the  exercise  of
      warrants held by Pillar S.A., (d) 20,000 shares issuable upon the exercise
      of warrants held by Pillar  S.A.R.L.,  (e) 37,500 shares issuable upon the
      exercise  of Class C  warrants  held by  Pillar  Investment  Limited,  (f)
      473,598  issuable  upon the exercise of advisory  warrants  held by Pillar
      Investment  Limited,  (g) 638,032  shares  issuable  upon the  exercise of
      placement  warrants held by Pillar  Investment  Limited,  (h) 5,243 shares
      issuable  upon the exercise of other  warrants  held by Pillar  Investment
      Limited,  (i) 462,800 shares held by Pillar  Investment  Limited,  and (j)
      9,000  shares  issuable  upon the  exercise of stock  options  held by Mr.
      Menhall.  Mr.  Menhall,  an affiliate of Pillar  Associated,  Pillar S.A.,
      Pillar  S.A.R.L.  and  Pillar  Investment  Limited,  may be  considered  a
      beneficial owner of the shares beneficially owned by such entities.

(11)  Includes (a) 37,500 shares  issuable upon the exercise of Class C warrants
      held by Pillar Investment Limited,  (c) 473,598 issuable upon the exercise
      of advisory warrants held by Pillar Investment Limited,


                                       44



<PAGE>

      (c) 638,032 shares  issuable upon the exercise of placement  warrants held
      by Pillar  Investment  Limited,  and (d) 5,243  shares  issuable  upon the
      exercise of other warrants held by Pillar Investment Limited.

(12)  Includes  1,125,880  shares  held by Nicris  Limited  and  234,764  shares
      issuable upon the exercise of Class B warrants held by Nicris Limited. Mr.
      Bin Laden,  a  controlling  stockholder  of Nicris,  may be  considered  a
      beneficial owner of the shares beneficially owned by such entity.

(13)  Includes  234,764  shares  issuable  upon the exercise of Class B warrants
      held by Nicris Limited.

(14)  Includes  42,878 shares of Series A Convertible  Preferred Stock which are
      convertible  into  1,008,894  shares of Common Stock of the Company.  This
      amount also includes  238,023 shares issuable upon the exercise of Class A
      warrants.

(15)  Includes  233,026  shares  issuable  upon the exercise of Class B warrants
      held by Faisal Finance Switzerland SA.

(16)  Includes  259,375  shares  issuable  upon the exercise of Class C warrants
      held by Finova Technology Finance Inc.

(17)  Includes  26,549 shares of Series A Convertible  Preferred Stock which are
      convertible  into  624,682  shares of Common  Stock of the  Company.  This
      amount also includes  42,153 shares  issuable upon the exercise of Class A
      warrants, 74,265 shares issuable upon the exercise of Class D warrants and
      129,000  shares  issuable upon  conversion of a portion of the  $6,000,000
      bank loan to the Company owned by this entity.


            The following  table sets forth certain  information  as of June 15,
1999 with  respect to the  beneficial  ownership  of shares of Common  Stock and
Series A  Convertible  Preferred  Stock by (i) the  directors of the Company and
(ii) the Chief Executive Officer and other Named Executive  Officers,  and (iii)
the  directors  and  executive  officers  of the  Company  as a group,  assuming
conversion  of all  convertible  debt or  preferred  stock and  exercise  of all
warrants and stock options by such person and only by such person.
<TABLE>
<CAPTION>

                                                                                                          Series A
                                                                Common Stock                     Convertible Preferred Stock
                                                                ------------                     ---------------------------
                                                   Amount and Nature of    Percent of      Amount and Nature of      Percent of
Name of Beneficial Owner                           Beneficial Ownership      Class         Beneficial Ownership         Class
DIRECTORS
<S>                                                     <C>                 <C>                <C>                     <C>
Arthur W. Berry.................................        8,442,556(2)        34.87%             255,381(3)              38.57%
Harold W. Purkey................................        4,924,415(4)        24.75%              46,677(5)               7.05%
Youssef El-Zein.................................        1,748,722(6)        10.25%                   0                    0
Nasser Menhall..................................        1,726,734(7)        10.14%                   0                    0
E. Andrews Grinstead, III ......................          678,176(8)         4.14%                   0                    0
Sudhir Agrawal..................................          519,116(9)         3.19%                   0                    0
Paul Z. Zamecnik................................          343,180(10)        2.17%                   0                    0
James B. Wyngaarden.............................           85,850(11)           *                    0                    0
Camille A. Chebeir..............................           25,000               *                    0                    0
H.F. Powell.....................................           18,750(12)           *                    0                    0
All directors and executive officers as a group
(10 persons)....................................       16,855,760(13)       54.36%             302,058                45.62%
</TABLE>

- -------------------

*     Less than 1%.

(1)   The number of shares  beneficially  owned by each  director and  executive
      officer is determined under rules  promulgated by the Commission,  and the
      information is not necessarily  indicative of beneficial ownership for any
      other purpose. Under such rules,  beneficial ownership includes any shares
      as to which the  individual  has sole or shared voting power or investment
      power and also any shares which

                                       45



<PAGE>

      the individual has the right to acquire within 60 days after June 15, 1999
      through the  exercise of any stock option or other  right.  The  inclusion
      herein of such shares,  however, does not constitute an admission that the
      named stockholder is a direct or indirect beneficial owner of such shares.
      Unless otherwise  indicated,  each person or entity named in the table has
      sole voting power and  investment  power (or shares such power with his or
      her spouse) with respect to all shares of capital stock listed as owned by
      such person or entity.

(2)   Includes  255,381 shares of Series A Convertible  Preferred Stock owned by
      six  investment  advisory  clients of Pecks,  which  clients would receive
      dividends  and the proceeds  from the sale of such shares.  Three of these
      clients are Delaware  State  Employees  Retirement  Fund,  General  Motors
      Employees  Domestic  Group Trust and  Declaration of Trust for the Defined
      Benefit  Plan of ICI  American  Holdings,  Inc.  These  shares of Series A
      Convertible  Preferred  Stock are  convertible  into  6,008,965  shares of
      Common  Stock of the Company.  This amount also  includes  762,415  shares
      issuable upon the exercise of Class A warrants and 420,172 shares issuable
      upon  the  exercise  of  Class D  warrants  held in the  aggregate  by the
      foregoing  entities.  This number also includes  1,250,000 shares issuable
      upon  conversion of a portion of the  $6,000,000  bank loan to the Company
      owned by certain of the  foregoing  entities.  Mr.  Berry,  a principal of
      Pecks,  may be  considered a beneficial  owner of the shares owned by such
      entities. Mr. Berry disclaims beneficial ownership of these shares.

(3)   Includes  255,381 shares of Series A Convertible  Preferred Stock owned by
      six  investment  advisory  clients of Pecks,  which  clients would receive
      dividends  and the proceeds  from the sale of such shares.  Three of these
      clients are Delaware  State  Employees  Retirement  Fund,  General  Motors
      Employees  Domestic  Group Trust and  Declaration of Trust for the Defined
      Benefit  Plan of ICI  American  Holdings,  Inc.  These  shares of Series A
      Convertible  Preferred  Stock are  convertible  into  6,008,965  shares of
      Common  Stock of the Company.  This amount also  includes  762,419  shares
      issuable upon the exercise of Class A warrants and 420,172 shares issuable
      upon  the  exercise  of  Class D  warrants  held in the  aggregate  by the
      foregoing  entities.  This number also includes  1,250,000 shares issuable
      upon  conversion of a portion of the  $6,000,000  bank loan to the Company
      owned by certain of the  foregoing  entities.  Mr.  Berry,  a principal of
      Pecks,  may be  considered a beneficial  owner of the shares owned by such
      entities. Mr. Berry disclaims beneficial ownership of these shares.

(4)   Includes (a) 796,259 shares of Common Stock owned by Forum Capital Markets
      LLC,  (b) 328,677  shares  issuable  upon the exercise of Class B warrants
      owned by Forum,  (c) 280,517 shares  issuable upon the exercise of Class C
      warrants owned by Forum,  (d) 408,112 shares issuable upon the exercise of
      Class A warrants owned by Forum,  (e) 61,568 shares issuable upon exercise
      of othr  warrants  held by  forum,  (f)  1,250,000  shares  issuable  upon
      conversion of Forum's  portion of the $6,000,000  bank loan to the Company
      and (g)  1,098,282  shares  issuable  upon  conversion of 46,677 shares of
      Series A  Convertible  Preferred  Stock  owned by Forum.  Mr.  Purkey,  an
      affiliate of Forum,  may be  considered  a beneficial  owner of the shares
      beneficially owned by such entity.

(5)   Consists of 46,667 shares of Series A Convertible Preferred Stock owned by
      Forum.  Mr. Purkey,  an affiliate of Forum, may be considered a beneficial
      owner of the shares beneficially owned by Forum.

(6)   Includes (a) 82,183 shares  issuable upon the exercise of warrants held by
      Mr. El-Zein, (b) 366 shares issuable upon the exercise of warrants held by
      Pillar  Associated,  (c)  20,000  shares  issuable  upon the  exercise  of
      warrants held by Pillar S.A., (d) 20,000 shares issuable upon the exercise
      of warrants held by Pillar  S.A.R.L.,  (e) 37,500 shares issuable upon the
      exercise  of Class C  warrants  held by  Pillar  Investment  Limited,  (f)
      473,598  issuable  upon the exercise of advisory  warrants  held by Pillar
      Investment  Limited,  (g) 638,032  shares  issuable  upon the  exercise of
      placement  warrants held by Pillar  Investment  Limited,  (h) 5,243 shares
      issuable  upon the exercise of other  warrants  held by Pillar  Investment
      Limited,  (i) 462,800 shares held by Pillar  Investment  Limited,  and (j)
      9,000  shares  issuable  upon the  exercise of stock  options  held by Mr.
      El-Zein.  Mr.  El-Zein,  an affiliate of Pillar  Associated,  Pillar S.A.,
      Pillar  S.A.R.L.  and  Pillar  Investment  Limited,  may be  considered  a
      beneficial owner of the shares beneficially owned by such entities.


                                       46



<PAGE>

(7)   Includes (a) 60,195 shares  issuable upon the exercise of warrants held by
      Mr. Menhall, (b) 366 shares issuable upon the exercise of warrants held by
      Pillar  Associated,  (c)  20,000  shares  issuable  upon the  exercise  of
      warrants held by Pillar S.A., (d) 20,000 shares issuable upon the exercise
      of warrants held by Pillar  S.A.R.L.,  (e) 37,500 shares issuable upon the
      exercise  of Class C  warrants  held by  Pillar  Investment  Limited,  (f)
      473,598  issuable  upon the exercise of advisory  warrants  held by Pillar
      Investment  Limited,  (g) 638,032  shares  issuable  upon the  exercise of
      placement  warrants held by Pillar  Investment  Limited,  (h) 5,243 shares
      issuable  upon the exercise of other  warrants  held by Pillar  Investment
      Limited,  (i) 462,800 shares held by Pillar  Investment  Limited,  and (j)
      9,000  shares  issuable  upon the  exercise of stock  options  held by Mr.
      Menhall.  Mr.  Menhall,  an affiliate of Pillar  Associated,  Pillar S.A.,
      Pillar  S.A.R.L.  and  Pillar  Investment  Limited,  may be  considered  a
      beneficial owner of the shares beneficially owned by such entities.

(8)   Includes  630,596 shares  subject to  outstanding  stock options which are
      exercisable within the 60-day period following June 15, 1999.

(9)   Includes  501,356 shares  subject to  outstanding  stock options which are
      exercisable within the 60-day period following June 15, 1999.

(10)  Includes (a) 50,750 shares subject to outstanding  stock options which are
      exercisable  within  the 60-day  period  following  June 15,  1999 and (b)
      31,250 shares issuable upon the exercise of Class C warrants.

(11)  Includes (a) 62,000 shares subject to outstanding  stock options which are
      exercisable  within the 60-day period following April 16, 1999 and (b) 700
      shares held by Mr. Wyngaarden's children.

(12)  Includes  18,750  shares  subject to  outstanding  stock options which are
      exercisable within 60-day period following June 15, 1999.

(13)  Securities owned by Pillar  Associated,  Pillar S.A., Pillar S.A.R.L.  and
      Pillar  Investment  Limited are included only once,  although such amounts
      were included above for both Messrs. El-Zein and Menhall.


                 CERTAIN RELATIONSHIPS AND RELATED TRANSACTIONS

            Since January 1, 1998, Hybridon has entered into or has been engaged
in  the  following  transactions  with  the  following  Hybridon  directors  and
officers,  stockholders  who  beneficially  own more than 5% of the  outstanding
Common Stock of Hybridon ("5% Stockholders"), and affiliates or immediate family
members of those directors, officers and 5% Stockholders.

TRANSACTIONS WITH PILLAR S.A. AND CERTAIN OF ITS AFFILIATES

            Hybridon  has entered into  certain  transactions  with Pillar S.A.,
Pillar Investment,  and Charles River Building Limited  Partnership,  the entity
which owned the Company's former  headquarters in Cambridge,  Massachusetts (the
"Cambridge  Landlord").  Pillar S.A. and Pillar  Investment  are  affiliates  of
Messrs.  El-Zein and Menhall, two directors of Hybridon.  The Cambridge Landlord
is an affiliate of Messrs. El-Zein and Menhall and Mohamed El-Khereiji, a former
director of  Hybridon.  The  following is a summary of those  transactions  that
relate to Hybridon's 1998 fiscal year.

            In 1998, Hybridon was a party to a consulting  agreement with Pillar
S.A.  dated  as of March 1,  1994  (the  "1994  Pillar  Consulting  Agreement"),
pursuant to which Pillar S.A.  provided  Hybridon  with  financial  advisory and
managerial services in connection with Hybridon's overseas operations, including
support services in connection with contracts and agreements. Under the terms of
the 1994 Pillar Consulting Agreement,  Hybridon paid Pillar S.A. consulting fees
of $60,000 per month and $23,000 per month for overhead  costs,  and  reimbursed
certain authorized  out-of-pocket expenses. The 1994 Pillar Consulting Agreement
expired on February 28, 1998.


                                       47



<PAGE>

            On July 8, 1995, Hybridon entered into an additional  agreement with
Pillar S.A. (the "Pillar Europe Agreement") pursuant to which Pillar S.A. agreed
for a period of two years to provide to Hybridon  certain  consulting,  advisory
and related services (in addition to the services to be provided pursuant to the
1994 Pillar  Consulting  Agreement) and serve as Hybridon's  exclusive  agent in
connection   with  potential   corporate   partnerships   in  Europe  and  as  a
non-exclusive  placement agent of Hybridon in connection with private placements
of securities of Hybridon.  On November 1, 1995, the Pillar Europe Agreement was
amended  to provide  that (1) Pillar  S.A.  would  cease to serve as  Hybridon's
executive agent in connection with potential  corporate  partnerships in Europe,
but would continue to serve as a  non-exclusive  agent in that  connection,  (2)
Pillar S.A. would receive a retainer of $26,470 per month for the balance of the
term of the Pillar  Europe  Agreement,  (3) the fees  provided for in the Pillar
Europe  Agreement  would only be  payable  to Pillar  S.A.  in  connection  with
potential  collaborations  with any  French  pharmaceutical  company  with which
Hybridon  engaged in  discussions  during the 12-month  period ended November 1,
1995 as a result of  introductions  by  Pillar  S.A.,  and (4) any  compensation
payable to Pillar S.A. in  connection  with its  services  with respect to other
corporate  collaborations or any placements of securities would be negotiated on
a  case-by-case  basis and would be subject to the  approval of the  independent
members of the Board of  Directors  of  Hybridon.  The Pillar  Europe  Agreement
expired on April 1, 1997.

            In 1998,  Hybridon  paid Pillar  Investment an aggregate of $300,000
under  these  agreements,  in the form of  150,000  shares of  Common  Stock and
warrants to purchase  37,500  shares of Common  Stock,  at an exercise  price of
$2.40 per share, subject to adjustment, in lieu of cash.

            Hybridon  has  retained  Pillar  Investment  as  placement  agent in
connection  with the private  placements  of  securities of Hybridon in offshore
transactions in reliance upon an exemption from registration  under Regulation S
(the "Regulation S Offerings")  promulgated under the Securities Act of 1933, as
amended (the "Securities  Act").  Pillar Investment  received fees consisting of
(1)  9%  of  the  gross   proceeds  of  each   Regulation  S  Offering,   (2)  a
non-accountable  expense allowance equal to 4% of those gross proceeds,  (3) the
right to  purchase,  for nominal  consideration,  warrants  to purchase  473,598
shares of Common  Stock,  at an  exercise  price of $2.40 per share,  subject to
adjustment  (the  "Pillar  Warrants"),  (4) the right to  purchase,  for nominal
consideration,  warrants to  purchase a number of shares of the Common  Stock of
Hybridon equal to 10% of the aggregate  number of shares of Common Stock sold by
Hybridon for which Pillar  Investment acted as placement  agent,  exercisable at
120% of the relevant  Common Stock  offering  price,  for a period of five years
(resulting,  as of the date hereof, in the right to receive warrants to purchase
638,032  shares  at  $2.40  per  share,  subject  to  adjustment),   and  (5)  a
consulting/restructuring  fee of  $960,000  payable in Common  Stock of Hybridon
valued at the  market  price and  payable  in three  equal  installments  as net
proceeds  of  $25,000,000,  $30,000,000  and  $35,000,000  are  received  in the
aggregate  from  private  placements  effected by Hybridon in 1998 to the extent
contemplated  by the  Consent  and Waiver  dated as of January 12, 1998 given by
certain beneficial holders of Hybridon's 9% convertible subordinated notes (the"
9% Notes"),  or otherwise to the extent  contemplated  by the  Placement  Agency
Agreement between Hybridon and Pillar Investment,  subject to Hybridon's receipt
of a fairness  opinion with regard  thereto.  In no event may Pillar  Investment
receive  compensation in excess of the level that was approved by the holders of
the 9% Notes.  Pillar  Investment  has received  $1,635,400  in cash pursuant to
these  arrangements  and Pillar warrants to purchase  1,111,630 shares of Common
Stock.

            In addition, in connection with the Regulation S Offerings, Hybridon
and Pillar Investment have entered into an advisory agreement dated May 5, 1998,
pursuant to which Pillar Investment acts as Hybridon's  non-exclusive  financial
advisor.  This  agreement  requires  that  Hybridon  pay an  affiliate of Pillar
Investment a monthly retainer of $5,000 (with a minimum  engagement of 24 months
beginning  on May 5, 1998),  and further  provides  that  Pillar  Investment  is
entitled  to receive  (1)  out-of-pocket  expenses,  (2)  subject to  Hybridon's
receipt of a fairness  opinion with respect  thereto,  300,000  shares of Common
Stock in connection with Pillar  Investment's  efforts in assisting  Hybridon in
restructuring its balance sheet, and (3) certain cash and equity success fees in
the  event  Pillar  Investment  assists  Hybridon  in  connection  with  certain
financial and strategic  transactions.  As of April 16, 1999, Hybridon issued to
Pillar  Investment  the  stipulated  300,000  shares of Common  Stock.  Hybridon
received a fairness opinion in connection with that issuance.



                                       48



<PAGE>

TRANSACTIONS WITH THE CAMBRIDGE LANDLORD

            From February 4, 1997 to September 16, 1998, Hybridon was a party to
a lease with the Cambridge Landlord for its Cambridge facilities (the "Cambridge
Lease") . The Cambridge  Lease  originally  provided for an annual rent equal to
$30 per square foot on a triple-net  basis  (meaning that the tenant pays taxes,
insurance, and operating costs) for the first five years, $33 per square foot on
a  triple-net  basis for the next five  years and the  greater of $30 per square
foot on a triple-net basis or the then-market  value of leased property for each
of the five-year  renewal  terms.  In  connection  with  Hybridon's  election to
acquire an interest in the Cambridge  Landlord,  as described  below, the annual
rent due under the Cambridge Lease was increased for the first five years of the
lease term to $38 per square  foot on a  triple-net  basis,  for the second five
years to $42 per square foot on a triple-net  basis and for the third five years
to $47 per square foot on a triple-net basis.

            On July 1, 1996, Hybridon elected to fund approximately $5.5 million
of the costs (primarily relating to tenant  improvements) of the construction of
the leased  premises  through  contributions  to the  capital  of the  Cambridge
Landlord  in  exchange  for a  limited  partnership  interest  in the  Cambridge
Landlord  (the  "Partnership  Interest").   The  Partnership  Interest  entitled
Hybridon to an approximately  32% interest in the Cambridge  Landlord.  Hybridon
had the right,  for a period of three years ending  February  2000,  to sell the
Partnership  Interest back to certain limited partners of the Cambridge Landlord
for a price equal to the greater of (1) the aggregate cash  contribution made by
Hybridon  to the  Cambridge  Landlord  or  (2)  the  fair  market  value  of the
Partnership Interest at the time.

            In 1997, Hybridon had on deposit with Bank fur  Vermogensanlagen und
Handel  ("BVH") the amount of  $1,034,618.  In  November  1997,  German  banking
authorities imposed a moratorium on BVH and closed BVH for business. Pursuant to
an agreement  dated November 28, 1997, the Cambridge  Landlord  agreed to assume
the risk for the BVH  deposit  and to pay to  Hybridon  the  amount of $75,000 a
month after each rent payment under the Cambridge Lease was made until such time
as $1,000,000 had been paid to Hybridon or the BVH deposit was released.

            In June 1998, Hybridon relocated its headquarters from the Cambridge
facility to its facility in Milford,  Massachusetts.  The Cambridge facility was
re-leased in September 1998 to a third party, subject to a sublease of a portion
of the facility.  As a result,  Hybridon  terminated the Cambridge Lease and was
relieved of its substantial lease obligations under the Cambridge Lease, subject
to a contingent  continuing  liability for any sublessee  defaults.  Further, in
November 1998  Hybridon  completed the sale of its  Partnership  Interest.  As a
result of these  transactions,  Hybridon received  $6,163,000 from the Cambridge
Landlord,  which included payment for the Partnership Interest,  the return of a
portion of the security  deposit required under the Cambridge Lease, and payment
in full of the BVH deposit.

TRANSACTIONS WITH FORUM CAPITAL MARKETS LLC AND PECKS MANAGEMENT PARTNERS LTD.

            In 1998,  Hybridon entered into certain  transactions with Forum, an
affiliate of Mr. Purkey,  a director of Hybridon,  and entities advised by Pecks
Management  Partners  Ltd.,  Mr. Berry,  a principal of Pecks,  is a director of
Hybridon.

            Hybridon  retained  Forum  as  a  placement  agent  of  Hybridon  in
connection  with  Hybridon's  1998 Regulation D offering of Series A Convertible
Preferred  Stock and Class D warrants in the U.S. Forum received as compensation
for its services as placement agent with regard to the Regulation D offering and
its assistance  with an exchange offer made by the Company to the holders of its
9%  Notes,  597,699  shares  of  Common  Stock and  warrants  (the  "1998  Forum
Warrants")  to purchase  prior to May 4, 2003 an aggregate of 609,194  shares of
Common Stock exercisable at $2.40 per share, in each case subject to adjustment.
In addition,  in consideration of the agreements made by Forum consenting to the
Regulation  D offering  and waiving  certain  obligations  of Hybridon to Forum,
Hybridon  agreed to amend Forum's warrant dated as of April 2, 1997, to purchase
up to 71,301 shares of Common Stock of Hybridon (the "1997 Forum  Warrant"),  to
change  the  exercise  price to $4.25 per  share,  subject  to  adjustment,  and
increase  the number of shares of Common  Stock  purchasable  upon  exercise  to
588,235,  in each case subject to adjustment,  and to provide that it may not be
exercised until May 5, 1999 and the  transactions  contemplated by those private
placements and by


                                       49

<PAGE>

the  exchange  offer  will not  trigger  any  anti-dilution  adjustments  to its
exercise  price or the  number  of  shares  of  Common  Stock  purchasable  upon
exercise.

            In November  1998,  Forum and  entities  advised by Pecks  purchased
Hybridon's  bank  loan.  In  connection  with  the  purchase  of the  loan,  the
purchasing  entities advanced an additional amount to Hybridon so as to increase
the outstanding  principal  amount of the loan to $6,000,000.  In addition,  the
purchasing entities agreed to amend the terms of the loan. This principal amount
of the loan and accrued but unpaid interest thereon is convertible,  in whole or
in part, at the lenders' option into Common Stock at a conversion price of $2.40
per share.

            In connection with the purchase of the loan, Forum received a fee of
$400,000,  which Forum has reinvested by purchasing from Hybridon 160,000 shares
of Common Stock and warrants to purchase an  additional  40,000 shares of Common
Stock at $3.00 per share. In addition, Forum received warrants exercisable until
maturity of the Loan to  purchase  133,333  shares of Common  Stock at $3.00 per
share.

            Hybridon  maintains  an  investment  account  at  Forest  Investment
Management LLC, an affiliate of Forum and Mr. Purkey.


OTHER TRANSACTIONS

            In March 1999, the Company entered into consulting arrangements with
each of Mr. Powell, Dr. Zamecnik and Dr. Wyngaarden  pursuant to which each such
person will act as a  consultant  to the Company for a two-year  period and will
receive a consulting fee of $20,000 per year for general consulting services. In
addition, each person will receive a consulting fee of $1,500 per day of on-site
consulting  services provided by such person at the Company's  corporate offices
(or at an  alternative  site agreed upon by the  parties),  and at the Company's
prior  request.   Additional  fees  for  special  projects  will  be  negotiated
separately  between  the  parties.  Each of Mr.  Powell,  Dr.  Zamecnik  and Dr.
Wyngaarden  also received  options to purchase  150,000  shares of the Company's
Common Stock at $2.00 per share; such options will vest over a two-year period.

            Certain  persons and entities (the  "Rightsholders"),  including Dr.
Zamecnik,  Pillar S.A., Pillar Limited,  Intercity  Holdings,  Mr. Bin Laden and
Nicris Limited,  are entitled to certain rights with respect to the registration
under the  Securities  Act of certain  shares of  Hybridon's  Common  Stock (the
"Registrable  Shares"),  including  shares of Common  Stock that may be acquired
pursuant to the exercise of options or warrants,  under the terms of  agreements
among  Hybridon  and the  Rightsholders  (the  "Registration  Agreements").  The
Registration Agreements generally provide that in the event Hybridon proposes to
register  any of its  securities  under the  Securities  Act at any  time,  with
certain  exceptions,  the Rightsholders,  including Pillar S.A., Pillar Limited,
Intercity  Holdings,  Mr. Bin Laden and Nicris  Limited,  but  excluding,  among
others,  Dr.  Zamecnik,  have the  additional  right under certain  Registration
Agreements to require Hybridon to prepare and file registration statements under
the  Securities  Act, if  Rightsholders  holding  specified  percentages  of the
Registrable Shares so request,  and Hybridon is required to use its best efforts
to effect that registration, subject to certain conditions and limitations.

            Hybridon believes that the terms of the transactions described above
were no less favorable than Hybridon could have obtained from unaffiliated third
parties.



                                       50




<PAGE>



                              SELLING STOCKHOLDERS


            The tables below set forth,  to the  knowledge of Hybridon,  certain
information  as of June 15, 1999 with respect to the Selling  Security  Holders.
The table entitled "Stockholders Selling Common Stock" includes information with
respect  to  Selling  Security  Holders  who are  selling  Common  Stock in this
offering.  The table entitled  "Stockholders  Selling  Preferred Stock" includes
information with respect to Selling  Security Holders who are selling  Preferred
Stock in this  offering.  Except as noted  below,  no  Selling  Security  Holder
selling  Common or Preferred  Stock in this  offering will own 1% or more of the
outstanding stock of Hybridon after the offering.

            Except as  described  below,  none of the Selling  Security  Holders
holds any position or office with, or has otherwise had a material  relationship
with, Hybridon within the past three years.

<TABLE>
<CAPTION>

                                                       Stockholders Selling Common Stock



                                          Number of Shares of                                              Number of Shares of
                                       Common Stock Beneficially            Number of Shares of         Common Stock Beneficially
                                                 Owned                             Common                         Owned
 Name of Selling Security Holder           Prior to Offering             Stock Included in Offering           After Offering
 -------------------------------          -------------------           ----------------------------         ---------------



<S>                                              <C>                               <C>                           <C>
Fouad M.O. Tawfig and  Hanan H.                  47,543                            6,250                         41,293
Zagzoug

Torben Duer                                      26,750                            18,750                         8,000

Thomas Fr. Duer                                  62,500                            62,500                           0

Darier Hentsch & Cie                            651,088                           651,088                           0

Finn Trunk Black                                 3,750                             3,750                            0

MM Pictet & Cie                                 750,000                           750,000                           0

Nicris Limited6                                1,360,644                         1,050,644                       310,000

Raji Abou Hadar                                  62,500                            62,500                           0

Intercity Ltd.6                                2,216,666                         1,875,000                       341,666

Clapham Investments Ltd.                        125,500                           125,000                          500

LGT Bank in Liechtenstein AG                    312,500                           312,500                           0

Participations Besancon                         125,000                           125,000                           0

Loxhall Limited                                  62,500                            62,500                           0

MicroTech Software a/s                           33,000                            31,250                         11,750

JSP Holdings ApS                                 24,500                            12,500                         12,000

Jan Poulson                                      18,750                            18,750                           0

Mr. Mohamad Hassan Abdul Ghani                   67,717                            67,717                           0

Dr. Khaled M.R. Abdul Ghani                     135,435                           135,435                           0

Mr. Imad Mustapha Mansour                        67,717                            67,717                           0

Mr. Malek Salam                                  88,033                            88,033                           0

Faisal Finance (Switzerland) S.A.              1,043,113                         1,009,779                        33,334

Mr. Guy Semon                                    22,149                            22,149                           0

Mrs. Francoise Semon                             22,149                            22,149                           0

Mr. Le Pelley Dumanoir                           22,149                            22,149                           0

Mr. Moh'd Abdo Sweidan                           67,119                            67,119                           0
</TABLE>


                                                            51



<PAGE>

<TABLE>
<CAPTION>

                                                       Stockholders Selling Common Stock


                                          Number of Shares of                                              Number of Shares of
                                       Common Stock Beneficially            Number of Shares of         Common Stock Beneficially
                                                 Owned                             Common                         Owned
 Name of Selling Security Holder           Prior to Offering             Stock Included in Offering           After Offering
 -------------------------------          -------------------           ----------------------------         ---------------

<S>                                              <C>                               <C>                              <C>
Mr. Isam Moh'd Khairy Kabbani                    67,119                            67,119                           0

Dr. Essam Ahmad Jawadm Alamdar                  201,357                           201,357                           0

Arab Islamic Bank (E.C.)                        503,394                           503,394                           0

Mr. Sobbi Adra                                   23,492                            23,492                           0

Mr. Mansour S.M.A. Al-Sharif                     65,972                            65,972                           0

Mr. Nafez M.M. Al-Jindi                          65,972                            65,972                           0

Solter Corporation                              217,345                           196,047                         21,298

Carset Overseas Corporation                     176,375                           176,375                           0

Mr. Ali A. Bajrai                               163,310                           163,310                           0

Pillar Investment Limited1                     1,617,173                         1,599,130                        18,043

Bioreliance Corporation                          16,697                            16,697                           0

Chestnut Partners                                62,500                            62,500                           0

Datamonitor                                      62,500                            62,500                           0

Finova Technology Finance, Inc.                 896,875                           896,875                           0

HPC America, Inc.                               218,750                           218,750                           0

Hyal Pharmaceutical Corporation                  17,500                            17,500                           0

SEIF Foundation2                                119,725                           119,725                           0

Janitronics                                      45,724                            45,724                           0

Kinetic Systems, Inc.                           163,238                           163,238                           0

Massachusetts Eye & Ear Infirmary                62,500                            62,500                           0

Norwegian Radium Hospital Research               37,500                            37,500                           0
Foundation

Susan and Anthony Russo                          62,500                            62,500                           0

Pharmakinetics Laboratories, Inc.                55,803                            55,803                           0

The Perkin Elmer Corporation                    205,377                           205,377                           0

Primedica Corporation                           364,418                           364,418                           0

Quintiles Transnational Corp.                   379,175                           379,175                           0

Siena Construction Corporation                   31,250                            31,250                           0

Sierra Biomedical, Inc.                         189,203                           189,203                           0

SP Pharmaceuticals LLC                          115,985                           115,985                           0

Southern Research Institute                      68,860                            68,860                           0

Transamerica Business Credit                    468,750                           468,750                           0
Corporation

Triumvirate Environmental, Inc.                  19,138                            19,138                           0

University of Kansas                             29,260                            29,260                           0

University of Massachusetts                      84,450                            84,450                           0

Paul C. Zamecnik and Mary V.                    343,180                           156,250                        186,930
Zamecnik, JTWROS2,6

Allstate Insurance Company                      487,095                           487,095                           0
</TABLE>


                                                            52



<PAGE>

<TABLE>
<CAPTION>

                                                       Stockholders Selling Common Stock

                                          Number of Shares of                                              Number of Shares of
                                       Common Stock Beneficially            Number of Shares of         Common Stock Beneficially
                                                 Owned                             Common                         Owned
 Name of Selling Security Holder           Prior to Offering             Stock Included in Offering           After Offering
 -------------------------------          -------------------           ----------------------------         ---------------

<S>                                             <C>                               <C>                               <C>
Angelo Gordon & Co., L.P.                       113,648                           113,648                           0

Michael Angelo, L.P.                            308,510                           308,510                           0

Ramius Fund Ltd.                                227,296                           227,296                           0

Raphael, L.P.                                   308,510                           308,510                           0

Medici Partners, L.P.                            97,396                            97,396                           0

CNA Income Shares, Inc.                         487,095                           487,095                           0

Forest Alternative Strategies Fund II,           25,971                            25,971                           0
L.P. Series A5I3

Forest Alternative Strategies Fund II,           12,997                            12,997                           0
L.P. Series A5M3

Forest Alternative Strategies Fund II,            3,897                             3,897                            0
L.P. Series B-33

Forest Fulcrum Ltd.3                            105,533                           105,533                           0

Forest Global Convertible Fund Series           156,182                           156,182                           0
A53

Forest Greyhound3                                32,481                            32,481                           0

Forest Performance Fund3                         20,470                            20,470                           0

LLT Ltd.3                                        25,971                            25,971                           0

Forum Capital Markets LLC4,6                   4,923,415                         3,634,855                      1,288,560

Providian Life & Health                         655,710                           655,710                           0

Monumental Life Insurance Co.                   534,227                           534,227                           0

The Guardian Pension Trust Fund                  97,396                            97,396                           0

Harris Investment Management                     90,147                            90,147                           0

Offshore Strategies Ltd.                        324,765                           324,765                           0

Libertyview Plus Fund                           162,381                           162,381                           0

Libertyview Fund LLC                             81,190                            81,190                           0

CPR (USA)                                       405,906                           405,906                           0

Lincoln National Life Insurance Co.            1,246,917                         1,246,917                          0

Lincoln National Convertible                    483,865                           483,865                           0
Securities  Fund

Weirton Trust                                   141,271                           141,271                           0

Walker Art Center                                53,595                            53,595                           0

United National Insurance Co.                    22,742                            22,742                           0

Equi Select Growth & Income Fund                162,383                           162,383                           0

Zazove Convertible Fund, L.P.                   155,436                           155,436                           0

Lois Wilkens                                     6,224                             6,224                            0

Winchester Convertible Plus Ltd.                126,647                           126,647                           0

Foundation Account  No. 1                        68,194                            68,194                           0

LLC Account No. 1                                32,481                            32,481                           0
</TABLE>


                                                            53



<PAGE>

<TABLE>
<CAPTION>

                                                       Stockholders Selling Common Stock

                                          Number of Shares of                                              Number of Shares of
                                       Common Stock Beneficially            Number of Shares of         Common Stock Beneficially
                                                 Owned                             Common                         Owned
 Name of Selling Security Holder           Prior to Offering             Stock Included in Offering           After Offering
 -------------------------------          -------------------           ----------------------------         ---------------

<S>                                              <C>                               <C>                              <C>
GPS Fund Limited                                 97,394                            97,394                           0

Telefix (First Delta)                            16,253                            16,253                           0

Guardian Life Insurance Co. of                 3,171,698                         3,171,698                          0
America

Declaration of Trust for the Defined            870,100                           741,100                        129,000
Benefits Plan of ICI America
Holdings, Inc.5

J.W. McConnell Family Foundation5               386,988                           329,988                         57,000

Delaware State Employees Retirement            2,473,698                         2,138,448                       355,250
Fund5,6

General Motors Employees Domestic              3,744,926                         3,179,301                       565,625
Group Trust5,6

Zeneca Holdings5                                583,370                           496,995                         86,375

Thermo Electron Balanced                        362,474                           305,724                         56,750
Investment Fund5

Tucker Anthony & R.L. Day, Inc.                  6,199                             6,199                            0
</TABLE>


NOTES:

1.   Mr.  Nasser  Menhall  and Mr.  Youssef  El-Zein,  members  of the  board of
     directors of Hybridon, are principals of Pillar Investment Limited.

2.   Dr.  Zamecnik is a member of the board of  directors  of Hybridon  and is a
     consultant to Hybridon.

3.   Harold W. Purkey,  a member of the board of  directors  of Hybridon,  is an
     affiliate of this Selling Security Holder.

4.   Harold W. Purkey,  a member of the board of  directors of Hybridon,  is the
     President and a 10% owner of Forum Capital Markets.

5.   Arthur W. Berry, a member of the board of directors of Hybridon,  serves as
     investment advisor to ICI American.

6.   These Selling Security Holders will beneficially own greater than 1% of the
     Company's  Common  Stock  after the  offering,  as follows:
<TABLE>
<CAPTION>

                                                   Percentage  of Outstanding
Selling Security Holder Stock           Common Stock  Beneficially Owned After  the Offering
- -----------------------------           ----------------------------------------------------

<S>                                                         <C>
Nicris Limited                                              1.96%
Intercity Ltd.                                              2.17%
Paul Zamecnik and Mary V.  Zamecnik, JTWROS                 1.18%
Forum Capital  Markets LLC                                  7.57%
DelawareState  Employees  Retirement Fund                   2.20%
General Motors  Employees  Domestic Group Trust             3.46%
</TABLE>

<TABLE>
<CAPTION>

                                              Number of Shares of
                                             Convertible Preferred            Number of Shares of             Number of Shares of
                                               Stock Beneficially            Convertible Preferred           Convertible Preferred
             Name of                             Owned Prior to                Stock Included in              Stock Beneficially
     Selling Security Holder                        Offering                       Offering                  Owned After Offering
   -------------------------------          -------------------           ----------------------------         ---------------

<S>                                                 <C>                            <C>                                <C>
Allstate Insurance Company                          16,750                         16,750                             0

Angelo Gordon & Co., L.P.                           3,908                           3,908                             0

Michael Angelo, L.P.                                10,609                         10,609                             0
</TABLE>


                                                                            54



<PAGE>

<TABLE>
<CAPTION>

                                                       Stockholders Selling Common Stock

                                          Number of Shares of                                              Number of Shares of
                                       Common Stock Beneficially            Number of Shares of         Common Stock Beneficially
                                                 Owned                             Common                         Owned
 Name of Selling Security Holder           Prior to Offering             Stock Included in Offering           After Offering
 -------------------------------          -------------------           ----------------------------         ---------------

<S>                                              <C>                             <C>                               <C>
Ramius Fund Ltd.                                 7,816                           7,816                             0

Raphael, L.P.                                    10,609                         10,609                             0

Medici Partners, L.P.                            3,349                           3,349                             0

CNA Income Shares, Inc.                          16,750                         16,750                             0

Forest Alternative Strategies Fund                893                             893                              0
II, L.P. Series A5I1

Forest Alternative Strategies Fund                447                             447                              0
II, L.P. Series A5M1

Forest Alternative Strategies Fund                134                             134                              0
II, L.P. Series B-31

Forest Fulcrum Ltd.1                             3,629                           3,629                             0

Forest Global Convertible Fund                   5,584                           5,584                             0
Series A51

Forest Greyhound1                                1,117                           1,117                             0

Forest Performance Fund1                          704                             704                              0

LLT Ltd.1                                         893                             893                              0

Forum Capital Markets LLC2                       46,677                         46,677                             0

Providian Life & Health                          21,563                         21,563                             0

Monumental Life Insurance Co.                    16,400                         16,400                             0

The Guardian Pension Trust Fund                  3,349                           3,349                             0

Harris Investment Management                     3,100                           3,100                             0

Offshore Strategies Ltd.                         11,168                         11,168                             0

Libertyview Plus Fund                            5,584                           5,584                             0

Libertyview Fund LLC                             2,792                           2,792                             0

CPR (USA)                                        13,958                         13,958                             0

Lincoln National Life Insurance Co.              42,878                         42,878                             0

Lincoln National Convertible                     16,639                         16,639                             0
Securities  Fund

Weirton Trust                                    4,858                           4,858                             0

Walker Art Center                                1,843                           1,843                             0

United National Insurance Co.                     782                             782                              0

Equi Select Growth & Income Fund                 5,584                           5,584                             0

Zazove Convertible Fund, L.P.                    5,345                           5,345                             0

Lois Wilkens                                      214                             214                              0

Winchester Convertible Plus Ltd.                 4,355                           4,355                             0

Foundation Account  No. 1                        2,345                           2,345                             0

LLC Account No. 1                                1,117                           1,117                             0

GPS Fund Limited                                 3,349                           3,349                             0

Telefix (First Delta)                             559                             559                              0

Guardian Life Insurance Co. of                  109,067                         109,067                            0
America

Declaration of Trust for the Defined             26,549                         26,549                             0
Benefits Plan of ICI America
Holdings, Inc.3
</TABLE>


                                                                   55



<PAGE>

<TABLE>
<CAPTION>


                                          Number of Shares of                                              Number of Shares of
                                       Common Stock Beneficially            Number of Shares of         Common Stock Beneficially
                                                 Owned                             Common                         Owned
 Name of Selling Security Holder           Prior to Offering             Stock Included in Offering           After Offering
 -------------------------------          -------------------           ----------------------------         ---------------

<S>                                              <C>                            <C>                                <C>
J.W. McConnell Family Foundation                 11,742                         11,742                             0

Delaware State Employees                         73,536                         73,536                             0
Retirement Fund3

General Motors Employees                        114,177                         114,177                            0
Domestic Group Trust3

Zeneca Holdings3                                 17,780                         17,780                             0

Thermo Electron Balanced                         11,597                         11,597                             0
Investment Fund3

</TABLE>



NOTES:

1.   Harold W. Purkey,  a member of the board of  directors  of Hybridon,  is an
     affiliate of this Selling Security Holder.

2.   Harold W. Purkey,  a member of the board of  directors of Hybridon,  is the
     President and a 10% owner of Forum Capital Markets.

3.   Arthur W. Berry,  a member of the board of directos of Hybridon,  serves as
     investment advisor to ICI American.

                                       56




<PAGE>

                          DESCRIPTION OF CAPITAL STOCK

            The  authorized  capital stock of Hybridon  consists of  100,000,000
shares of Common Stock and 5,000,000  shares of preferred  stock, par value $.01
per share (the  "Preferred  Stock"),  of which 1,500,000 have been designated as
Convertible Preferred Stock. On June 15, 1999, there were issued and outstanding
15,766,825  shares of Common Stock and 662,099 shares of  Convertible  Preferred
Stock.

            There  follows a brief  summary of the terms of the Common Stock and
the Convertible  Preferred  Stock. For further  information  please refer to the
Restated Certificate of Incorporation of Hybridon,  including the Certificate of
Designation for the Series A Convertible  Preferred  Stock (the  "Certificate of
Designation"), which is filed as an exhibit to the Registration Statement.


COMMON STOCK

            Holders of Common Stock are entitled to one vote for each share held
on all matters  submitted to a vote of  stockholders  and do not have cumulative
voting rights. Accordingly,  holders of a majority of the shares of Common Stock
entitled to vote in any  election of  directors  may elect all of the  directors
standing for election.  Holders of Common Stock are entitled to receive  ratably
such any dividends  declared by the Board of Directors out of legally  available
funds,  subject to any  preferential  dividend  rights of the Preferred Stock or
other securities.  Upon the liquidation,  dissolution or winding up of Hybridon,
the holders of Common  Stock are  entitled to receive  ratably the net assets of
Hybridon  available  after the  payment of all debts and other  liabilities  and
subject  to the  prior  rights  of any  outstanding  Preferred  Stock and to the
Liquidation Put Right  described in the next paragraph.  Holders of Common Stock
have no preemptive,  subscription,  redemption or conversion rights. The rights,
preferences and privileges of holders of Common Stock are subject to, and may be
adversely  affected  by,  the  rights of the  holders of shares of any series of
Preferred  Stock that Hybridon may  designate  and issue in the future,  and the
rights of creditors of Hybridon.

            Pursuant to the terms of the Unit  Purchase  Agreement,  the initial
purchasers of certain of the shares of Common Stock sold in the Regulation S and
the Regulation D Offerings (those shares, the "Put Shares; those purchasers, the
"Liquidation  Put Holders")  have the right to put those shares back to Hybridon
upon the  liquidation  of Hybridon,  but only after all other  indebtedness  and
obligations  of  Hybridon  and all rights of any  holders of any  capital  stock
ranking  prior and senior to the Common Stock with respect to  liquidation  have
been satisfied in full (that right, the "Liquidation  Put"). The Liquidation Put
is not transferrable,  and therefore purchasers of Common Stock pursuant to this
Prospectus  will not be able to exercise  the  Liquidation  Put with  respect to
those  shares.  Any  Liquidation  Put  Holders  that have not sold or  otherwise
transferred  any Put Shares will,  however,  be able to exercise the Liquidation
Put  with  respect  to  those  Put  Shares  upon  a  liquidation   of  Hybridon.
Consequently,  in the event of  liquidation  of  Hybridon,  holders of shares of
Common  Stock that are not  subject  to the  Liquidation  Put right may  receive
smaller  liquidation  distributions  per  share  than  they  would  have  had no
Liquidation  Put Holders  exercised  the  Liquidation  Put. As of June 15, 1999,
there were 9,597,476 Put Shares outstanding.


PREFERRED STOCK

            Under the terms of the Restated  Certificate of  Incorporation,  the
Board of Directors is authorized,  subject to any limitations prescribed by law,
without stockholder approval, to issue up to 5,000,000 shares of Preferred Stock
in  one  or  more  series  with  such  rights,   preferences,   privileges   and
restrictions,  including  voting rights,  dividend  rights,  conversion  rights,
redemption  privileges and  liquidation  preferences,  as the Board of Directors
determines.


                                       57




<PAGE>

SERIES A CONVERTIBLE PREFERRED STOCK

            Dividends.  Each  share of  Series  A  Convertible  Preferred  Stock
("Convertible  Preferred Stock") is entitled to receive  cumulative  semi-annual
dividends  payable,  at the option of Hybridon,  in cash or additional shares of
Convertible  Preferred  Stock,  at the rate of 6.5% per annum plus  accrued  but
unpaid  dividends.   Dividends  accrue  from  the  date  of  issuance  and  paid
semi-annually on April 1 and October 1 of each year or, if any such day is not a
business day, on the next  business day.  Dividends are paid, at the election of
Hybridon, either in cash or additional shares of Convertible Preferred Stock. In
calculating the number of shares of Convertible  Preferred Stock to be paid with
respect to each dividend,  the Convertible  Preferred Stock is valued at $100.00
per share  (subject  to  appropriate  adjustment  to  reflect  any stock  split,
combination,  reclassification  or reorganization  of the Convertible  Preferred
Stock).

            Liquidation   Preference.   In  the  event  of  a  (1)  liquidation,
dissolution or winding up of Hybridon,  whether voluntary or involuntary,  (2) a
sale or other disposition of all or substantially all of the assets of Hybridon,
or  (3)  any  consolidation,   merger,  combination,   reorganization  or  other
transaction  in  which  Hybridon  is  not  the  surviving  entity  or  if  stock
constituting  more  than 50% of  Hybridon's  voting  power is  exchanged  for or
changed into stock or securities of another entity,  cash, or any other property
(a  "Merger  Transaction")  (items  (1),  (2)  and (3) of  this  sentence  being
collectively referred to as a "Liquidation  Event"),  after payment of debts and
other  liabilities of Hybridon,  the holders of shares of Convertible  Preferred
Stock will be entitled to be paid out of Hybridon's available assets, before any
payment to holders of shares ranking junior to the Convertible  Preferred Stock,
an  amount  equal  to  the  Dividend  Base  Amount.  In  the  case  of a  Merger
Transaction,  however,  this payment may be made in cash, property or securities
of the entity surviving the Merger  Transaction.  If upon any Liquidation Event,
whether voluntary or involuntary, the assets to be distributed to the holders of
the Convertible  Preferred Stock are  insufficient to permit the payment to such
shareholders  of the full amount owed, then all of Hybridon's  available  assets
will be distributed  ratably to the holders of the Convertible  Preferred Stock.
All  shares  of  Convertible  Preferred  Stock  rank,  as to  payment  upon  the
occurrence of any  Liquidation  Event,  senior to the Common Stock and senior to
all other series of  Preferred  Stock,  unless the terms of any series  provides
otherwise.

            Right  of  Conversion.  Commencing  after  May 5,  1999,  shares  of
Convertible  Preferred Stock will be  convertible,  at the option of the holder,
into  shares of Common  Stock or other  securities  and  property.  The  initial
conversion  price per share of Common Stock (the  "Conversion  Price") is $4.25,
and is subject to adjustment as described below. The rate at which each share of
Convertible  Preferred  Stock is  convertible at any time into Common Stock (the
"Conversion  Rate") will be determined by dividing the then-existing  Conversion
Price into the "Dividend Base Amount" of a share of Convertible Preferred Stock,
which is equal to $100 plus accrued but unpaid dividends  (subject to adjustment
to reflect any stock split, combination,  reclassification, or reorganization of
the Convertible Preferred Stock).

            Adjustment of Conversion  Rate and Conversion  Price. As of June 15,
1999,  each  share  of  Convertible   Preferred   Stock  was  convertible   into
approximately  23.53 shares of Common  Stock.  In order to preserve the economic
value of shares of Convertible  Preferred  Stock,  the Conversion  Price will be
adjusted if Hybridon does the following;

     o    pays a dividend or makes a distribution  on any class of capital stock
          in shares of its Common Stock;

     o    subdivides  its  outstanding  Common  Stock  into a greater  number of
          shares;

     o    combines its outstanding Common Stock into a smaller number of shares;

     o    issues  shares of Common  Stock or  Preferred  Stock to any  holder of
          Common  Stock or Preferred  Stock  rights to acquire  Shares of Common
          Stock or  Preferred  Stock at a price per share  less than the  market
          price (as defined);


                                       58




<PAGE>

     o    pays or distributes to the holders of Common Stock or Preferred  Stock
          assets,  properties,  or rights to acquire Hybridon Capital Stock at a
          price per share less than the market price; or

     o    makes a distribution consistently solely of cash to the holders of any
          class of capital stock where,  during a specified 12 month period, the
          cash  distribution  exceeds 10% of the product of the market  price of
          the Common Stock multiplied by the total outstanding Common Stock.

            Exceptions to Adjustments.  No adjustment will,  however, be made to
either the Conversion Rate or the Conversion Price for issuances of Common Stock
or Preferred  Stock or cash paid to holders of shares of  Convertible  Preferred
Stock (1) as payment for accrued  dividends or (2) as a mandatory  conversion or
mandatory redemption payment as described below.

            Other  Changes in  Conversion  Rate.  Hybridon from time to time may
increase the Conversion  Rate by any amount for any period of time if the period
is at  least 20 days and if the  increase  is  irrevocable  during  the  period.
Whenever the Conversion  Rate is so increased,  Hybridon will notify  registered
holders.

            Hybridon may also increase the Conversion  Rate in order to avoid or
diminish any income tax to holders of Common Stock  resulting  from any dividend
or  distribution  of stock or  issuance  of rights or  warrants  to  purchase or
subscribe for stock or from any event treated as such for income tax purposes.

            The  Conversion  Price may not be  adjusted  to an amount  less than
$.001 per  share,  the  current  par value of the  Common  Stock  into which the
Convertible Preferred Stock is convertible.

            Mandatory  Conversion  and  Redemption.  Upon  giving  notice to the
holders of the Convertible  Preferred Stock,  Hybridon may, at its option, cause
the  Convertible  Preferred  Stock to be converted in whole or in part, on a pro
rata basis,  into shares of Common Stock using a Conversion Price equal to $4.00
if the  closing  bid price of the Common  Stock  equals or  exceeds  250% of the
Conversion  Price for at least 20 trading  days in any period of 30  consecutive
trading days.

            At any time after April 1, 2000, Hybridon may, at its option, redeem
the Convertible Preferred Stock for cash equal to the Dividend Base Amount.

            Class Voting  Rights.  Hybridon shall not,  without the  affirmative
vote or consent  of the  holders  of at least 50% of all  outstanding  shares of
Convertible  Preferred Stock,  voting separately as a class, (1) amend, alter or
repeal any provision of the Restated  Certificate of  Incorporation or Bylaws so
as adversely to affect the rights of the  Convertible  Preferred  Stock  (except
that the  issuance  of  securities  ranking  prior to, or pari passu  with,  the
Convertible  Preferred Stock (A) upon a Liquidation Event or (B) with respect to
the payment of  dividends  or  distributions  will not be  considered  to affect
adversely  the  relative  rights of the  Convertible  Preferred  Stock),  or (2)
authorize  or issue,  or  increase  the  authorized  amount of, the  Convertible
Preferred  Stock,  other  than  the  Convertible  Preferred  Stock  issuable  as
dividends on the Convertible Preferred Stock.

            Preemptive Rights.  The Convertible  Preferred Stock is not entitled
to any  preemptive  or  subscription  rights in  respect  of any  securities  of
Hybridon.

            Restrictions  on Change of Control.  So long as any 9% Notes  remain
outstanding,  no  holder of any  shares of  Convertible  Preferred  Stock  will,
without the prior written  consent of Hybridon,  be granted  voting  rights,  be
entitled  to receive  any voting  securities  of  Hybridon,  or be  entitled  to
exercise any conversion rights if that could, in Hybridon's reasonable judgment,
either  alone or in  conjunction  with other  issuances  or  holdings of capital
stock,  warrants or  convertible  securities of Hybridon,  result in a Change of
Control (as defined in the Indenture).



                                       59




<PAGE>

                          TRANSFER AGENT AND REGISTRAR

            The transfer agent and registrar for the Common Stock is ChaseMellon
Shareholder Services LLC.


           DELAWARE LAW AND CERTAIN PROVISIONS OF HYBRIDON'S RESTATED
             CERTIFICATE OF INCORPORATION, BY-LAWS AND INDEBTEDNESS

            Hybridon is subject to the provisions of Section 203 of the Delaware
General  Corporation Law, which prohibits a publicly-held  Delaware  corporation
from engaging in a "business combination" with an "interested stockholder" for a
period of three  years  after the date of the  transaction  in which the  person
became an interested stockholder, unless the business combination is approved in
a prescribed manner. A "business  combination" includes mergers, asset sales and
other   transactions   resulting  in  a  financial  benefit  to  the  interested
stockholder.  Subject to certain  exceptions,  an "interested  stockholder" is a
person who, together with affiliates and associates, owns, or within three years
did own, 15% or more of the  corporation's  voting stock.  The existence of this
provision could deter certain business combinations, including transactions that
might otherwise  result in holders of voting stock being paid a premium over the
market price for their shares.

            The Restated Certificate of Incorporation  provides for the division
of the  Board  of  Directors  into  three  classes  as  nearly  equal in size as
possible,  with the classes having staggered three-year terms. In addition,  the
Restated  Certificate  of  Incorporation  provides that directors may be removed
only for cause by the affirmative  vote of the holders of at least two-thirds of
the shares of capital stock entitled to vote. Under the Restated  Certificate of
Incorporation,  any  vacancy  on the  Board  of  Directors,  however  occurring,
including a vacancy  resulting from an enlargement of the Board, may filled only
by vote of a majority of the directors then in office. The classification of the
Board of Directors and the  limitations  on the removal of directors and filling
of  vacancies  could have the effect of making it more  difficult  for anyone to
acquire, or of discouraging anyone from acquiring, control of Hybridon.

            The Restated  Certificate  of  Incorporation  also requires that any
action  required or permitted to be taken by the  stockholders of Hybridon at an
annual  meeting or special  meeting of  stockholders  may be taken only if it is
properly  brought  before that meeting and may not be taken by written action in
lieu of a meeting and will require reasonable advance notice by a stockholder of
a proposal or director  nomination which that stockholder  desires to present at
any annual or special  meeting of  stockholders.  The  Restated  Certificate  of
Incorporation  further provides that special meetings of the stockholders may be
called  only by the Chief  Executive  Officer  or,  if none,  the  President  of
Hybridon,  or by the Board of Directors.  Under Hybridon's By-Laws, in order for
any matter to be considered  "properly  brought" before a meeting, a stockholder
must comply with certain requirements  regarding advance notice to Hybridon. The
foregoing   provisions  could  have  the  effect  of  delaying  until  the  next
stockholders  meeting  any given  stockholder  action,  even  though it might be
favored by the holders of a majority of the  outstanding  voting  securities  of
Hybridon.  These provisions may also discourage any person or entity from making
a tender offer for Shares of Common Stock,  because such person or entity,  even
if it acquired a majority of the  outstanding  voting  securities  of  Hybridon,
would be able to take action as a stockholder (such as electing new directors or
approving  a merger)  only at a duly  called  stockholders  meeting,  and not by
written consent.

            The Delaware  General  Corporation  Law provides  generally that the
affirmative  vote of a majority of the shares  entitled to vote on any matter is
required  to amend a  corporation's  certificate  of  incorporation  or by-laws,
unless a corporation's certificate of incorporation or by-law requires a greater
percentage.  The Restated  Certificate of Incorporation  and the By-Laws require
the  affirmative  vote of the  holders  of at least 75% of the shares of capital
stock of Hybridon issued and outstanding and entitled to vote to amend or repeal
any of the provisions described in the prior two paragraphs. Moreover, the Board
of Directors has the authority,  without further action by the stockholders,  to
fix the rights and  preferences  of, and to issue shares of, any Preferred Stock
other than the Convertible Preferred Stock.


                                       60




<PAGE>

            In  addition  to these  provisions  of Delaware  law,  the  Restated
Certificate  of  Incorporation,   and  the  ByLaws,   the  terms  of  Hybridon's
outstanding  9% Notes,  which were issued in the  aggregate  original  principal
amount of $50.0  million and of which  approximately  $1.3  million in principal
amount  remains  outstanding,  require  Hybridon,  upon a Change of  Control  of
Hybridon (as defined in the indenture for the 9% Notes),  to offer to repurchase
the 9%  Notes  at a  repurchase  price  equal  to 150% of the  principal  amount
thereof,  plus  accrued  and unpaid  interest  to the date of  repurchase.  This
provision,   together  with  the  provisions  of  the  Restated  Certificate  of
Incorporation  described above and other provisions of the Restated  Certificate
of Incorporation, may have the effect of deterring hostile takeovers or delaying
or  preventing   changes  in  control  or  management  of  Hybridon,   including
transactions in which  stockholders  might otherwise receive a premium for their
shares over then current market prices. In addition,  these provisions may limit
the ability of stockholders to approve  transactions that they may deem to be in
their best interests.


                              PLAN OF DISTRIBUTION

            The  Securities  may be  sold  from  time  to  time  by the  Selling
Shareholders  or their  pledgees,  donees,  transferees  or other  successors in
interest. Sales of the Securities may be effected on the NASD OTC Bulletin Board
or in  negotiated  transactions  at prices  then  prevailing  or  related to the
then-current market price, or at negotiated prices.

            The Securities may be sold directly or through brokers or dealers by
means of one or more of the  following  methods:  (i) block  trades in which the
broker or dealer  attempts to sell shares as agent but may position and resell a
portion of the block as principal to facilitate the transaction;  (ii) purchases
by a broker or dealer as principal  and resales by that broker or dealer for its
own account pursuant to this  Prospectus,  including resale to another broker or
dealer; and (iii) ordinary brokerage  transactions and transactions in which the
broker solicits  purchasers.  In effecting sales, brokers and dealers engaged by
Selling   Security   Holders  may  arrange  for  other  brokers  or  dealers  to
participate.  Brokers or dealers  may  receive  commissions  or  discounts  from
Selling Security Holders (or, if any such broker or dealer acts as agent for the
purchaser of such Securities,  from such purchaser) in amounts to be negotiated.
A broker-dealers may agree with the Selling Security Holders to sell a specified
number of  Securities at a stipulated  price per share,  and, to the extent that
broker-dealer  is  unable to do so  acting  as agent  for the  Selling  Security
Holders, to purchase as principal any unsold Securities at the price required to
fulfill  the   broker-dealer   commitment  to  the  Selling  Security   Holders.
Broker-dealers  who acquire  Securities as principal may thereafter resell those
Securities.

            The Selling Security Holders and any broker-dealers participating in
distribution of the Securities may be deemed  "underwriters"  within the meaning
of Section 2(11) of the Securities Act, and any profit on the sale of Securities
by the  Selling  Security  Holders and any  commissions  or  discounts  given to
broker-dealers  may be deemed  underwriting  commissions or discounts  under the
Securities  Act.  In  addition,  any of the  Securities  that  qualify  for sale
pursuant  to Rule 144 may be sold under Rule 144 rather  than  pursuant  to this
Prospectus.

            Hybridon  has agreed to  indemnify  certain of the Selling  Security
Holders,  each  underwriter  of  certain  of the  Securities,  and  each  person
controlling  certain of the  Selling  Security  Holders  within  the  meaning of
Section 15 of the Securities Act, against certain liabilities in connection with
the offer and sale of the Securities, including liabilities under the Securities
Act,  and to  contribute  to payments  those  persons may be required to make in
respect of such liabilities. Certain of the Selling Security Holders have agreed
to indemnify, in certain circumstances,  Hybridon against certain liabilities in
connection  with the offer  and sale of the  Securities,  including  liabilities
under the Securities Act, and to contribute to payments Hybridon may be required
to make in respect thereof.


                                  LEGAL MATTERS

            The validity of the Securities  offered by this  Prospectus  will be
passed upon for Hybridon by Kramer Levin  Naftalis & Frankel LLP, New York,  New
York.

                                       61




<PAGE>

                                     EXPERTS

            The consolidated financial statements of Hybridon as of December 31,
1996,  1997, and 1998 and for each of the years in the  three-year  period ended
December 31, 1998 included in this Prospectus and elsewhere in the  Registration
Statement  have  been  audited  by  Arthur  Andersen  LLP,   independent  public
accountants,  as indicated in their  report with respect  thereto,  which report
includes a paragraph  stating that there is substantial  doubt about  Hybridon's
ability to continue as a going concern, and are included herein in reliance upon
the authority of said firm as experts in accounting and auditing.

                                       62




<PAGE>

                         HYBRIDON, INC. AND SUBSIDIARIES
                   INDEX TO CONSOLIDATED FINANCIAL STATEMENTS
<TABLE>
<CAPTION>

                                                                                          PAGE
<S>                                                                                        <C>
REPORT OF INDEPENDENT PUBLIC ACCOUNTANTS                                                   F-2

CONSOLIDATED BALANCE SHEETS AS OF DECEMBER 31, 1997,
DECEMBER 31, 1998 AND MARCH 31, 1999 (UNAUDITED)                                           F-3

CONSOLIDATED  STATEMENTS  OF OPERATIONS  FOR THE YEARS ENDED  DECEMBER 31, 1996,
DECEMBER 31, 1997 AND DECEMBER 31, 1998 AND FOR THE THREE MONTHS ENDED
MARCH 31, 1998 AND MARCH 31, 1999 (UNAUDITED)                                              F-4

CONSOLIDATED  STATEMENTS OF  STOCKHOLDERS'  EQUITY (DEFICIT) FOR THE YEARS ENDED
DECEMBER  31,  1996,  DECEMBER  31, 1997 AND DECEMBER 31, 1998 AND FOR THE THREE
MONTHS
ENDED MARCH 31, 1999 (UNAUDITED)                                                           F-5

CONSOLIDATED  STATEMENTS  OF CASH FLOWS FOR THE YEARS ENDED  DECEMBER  31, 1996,
DECEMBER 31, 1997 AND DECEMBER 31, 1998 AND FOR THE THREE MONTHS ENDED
MARCH 31, 1998 AND MARCH 31, 1999 (UNAUDITED)                                              F-7

NOTES TO CONSOLIDATED FINANCIAL STATEMENTS                                                 F-8

</TABLE>




                                       F-1



<PAGE>

                    REPORT OF INDEPENDENT PUBLIC ACCOUNTANTS


To Hybridon, Inc.:

We have audited the accompanying  consolidated balance sheets of Hybridon,  Inc.
(a Delaware  corporation) and subsidiaries as of December 31, 1997 and 1998, and
the  related  consolidated   statements  of  operations,   stockholders'  equity
(deficit)  and cash  flows  for each of the  three  years  in the  period  ended
December 31, 1998.  These  financial  statements are the  responsibility  of the
Company's  management.  Our  responsibility  is to  express  an opinion on these
financial statements based on our audits.

We  conducted  our  audits  in  accordance  with  generally   accepted  auditing
standards.  Those standards require that we plan and perform the audit to obtain
reasonable assurance about whether the financial statements are free of material
misstatement.  An audit includes examining, on a test basis, evidence supporting
the amounts and disclosures in the financial statements.  An audit also includes
assessing the  accounting  principles  used and  significant  estimates  made by
management,  as well as evaluating the overall financial statement presentation.
We believe that our audits provide a reasonable basis for our opinion.

In our opinion,  the financial  statements  referred to above present fairly, in
all material respects, the financial position of Hybridon, Inc. and subsidiaries
as of  December  31,  1997  and  1998  and the  consolidated  results  of  their
operations  and their cash flows for each of the three years in the period ended
December 31, 1998, in conformity with generally accepted accounting principles.

The accompanying  consolidated  financial statements have been prepared assuming
that the Company will continue as a going concern. Since inception,  the Company
has incurred significant losses which it has funded through the issuance of debt
and equity  securities and through research and development  collaborations  and
licensing  agreements.  The Company  expects such  resources to fund  operations
through May 1999.  There is  substantial  doubt about the  Company's  ability to
continue as a going concern.  See Note 1 for  management's  plans. The financial
statements do not include any adjustments  that might result from the outcome of
this uncertainty.



                                                    /s/ ARTHUR ANDERSEN LLP



Boston, Massachusetts
February 19, 1999  (except with respect to the matter  discussed in Note 7(b) as
to which the date is April 15, 1999)


                                       F-2



<PAGE>

<TABLE>
<CAPTION>


                                                              HYBRIDON, INC. AND SUBSIDIARIES
                                                                CONSOLIDATED BALANCE SHEETS

                                                                              ASSETS

                                                                                                                          March 31,
                                                                                          December 31,                      1999
                                                                               1997                       1998          (unaudited)
<S>                                                                            <C>                 <C>                <C>
CURRENT ASSETS:
      Cash and cash equivalents                                                $     2,202,202    $     5,607,882    $    2,461,184
      Accounts receivable                                                              529,702          1,175,441         1,278,492
      Prepaid expenses and other current assets                                      1,005,825            110,827           105,421
                                                                               ---------------    ----------------   --------------

            Total current assets                                                     3,737,729          6,894,150         3,845,097
                                                                               ---------------    ----------------   --------------

PROPERTY AND EQUIPMENT, AT COST:
      Leasehold improvements                                                        16,027,734         11,127,035        11,127,035
      Laboratory and other equipment                                                14,288,083         11,432,435        11,430,255
                                                                               ---------------    ---------------    --------------
                                                                                    30,315,817         22,559,470        22,557,290

      Less--Accumulated depreciation and amortization                               11,085,013         13,788,979       14,479,810
                                                                               ---------------    ---------------    --------------

                                                                                    19,230,804          8,770,491         8,077,480
                                                                               ---------------    ---------------    --------------
OTHER ASSETS:
      Deferred financing costs and other assets                                      3,354,767            612,374           585,390
      Note receivable from officer                                                     247,250            258,650           261,500
      Restricted cash                                                                3,050,982                  -                 -
      Investment in real estate partnership                                          5,450,000                  -                 -
                                                                               ===============    ===============    ==============
                                                                                    12,102,999            871,024           846,890
                                                                               ---------------    ---------------    --------------

                                                                               $    35,071,532    $    16,535,665    $   12,769,467
                                                                                ===============   ================    =============


              LIABILITIES AND STOCKHOLDERS' EQUITY (DEFICIT)

Current Liabilities:
      Current portion of long-term debt                                        $     7,868,474    $     6,070,951    $    6,073,283
      Accounts payable                                                               8,051,817          2,368,163         1,838,983
      Accrued expenses                                                              11,917,298          4,068,679         3,753,129
                                                                               ---------------    ----------------   --------------

                        Total current liabilities                                   27,837,589         12,507,793        11,665,395
                                                                               ---------------    ----------------   --------------

LONG-TERM DEBT, NET OF CURRENT PORTION                                               3,282,123            473,094           453,875
                                                                               ---------------    ----------------   --------------

9% CONVERTIBLE SUBORDINATED NOTES PAYABLE                                           50,000,000          1,306,000        1,306,000
                                                                               ---------------    ----------------   --------------

COMMITMENTS AND CONTINGENCIES (Notes 11 and 16)

STOCKHOLDERS' EQUITY (DEFICIT):
      Preferred stock, $.01 par value-
           Authorized--5,000,000 shares
           Series A convertible preferred stock-
                  Designated--1,500,000 shares
                  Issued and outstanding--641,259 shares                                    -              6,413             6,413
                                          (Liquidation preference of $65,168,048 at
                                           March 31, 1999)
      Common stock, $.001 par value-
            Authorized--100,000,000 shares
            Issued and outstanding--5,059,650 shares at December 31,
                  1997 and 15,304,825 at December 31, 1998
                  and March 31, 1999 (unaudited), respectively                           5,060             15,305            15,305
      Additional paid-in capital                                                   173,695,698        241,632,024       242,674,076
      Accumulated deficit                                                         (218,655,101)      (238,447,837)     (242,456,081)
      Deferred compensation                                                         (1,093,837)          (957,127)         (895,516)
                                                                                 -------------     --------------    --------------
            Total stockholders' (deficit) equity                                   (46,048,180)         2,248,778         (655,803)
                                                                                 -------------     --------------    --------------
                                                                                 $  35,071,532     $   16,535,665   $    12,769,467
                                                                                 =============     ==============    ==============
</TABLE>

The  accompanying  notes are an integral  part of these  consolidated  financial
statements.

                                                                           F-3



<PAGE>



                                    HYBRIDON, INC. AND SUBSIDIARIES

                                 CONSOLIDATED STATEMENTS OF OPERATIONS

<TABLE>
<CAPTION>

                                                                      Years Ended                           Three Months Ended
                                                                          December 31,                            March 31,

                                                         1996                1997            1998            1998         1999
                                                                                                               (unaudited)
<S>                                                     <C>               <C>            <C>              <C>          <C>
REVENUES:
      Product and service                               $1,080,175        $1,876,862     $3,253,879       $825,069     $1,529,854
      Research and development                           1,419,389           945,000      1,099,915        150,000        150,000
      Royalty and other income                              62,321            48,000              -              -         40,225
      Interest                                           1,446,762         1,079,122        148,067         17,845         52,801
                                                        ----------      ------------   ------------   ------------  -------------

                                                         4,008,647         3,948,984      4,501,861        992,914      1,772,880
                                                        ----------      ------------   ------------  --------------  ------------

OPERATING EXPENSES:
      Research and development                          39,390,525        46,827,915     20,977,370      6,402,537      3,447,278
      General and administrative                        11,346,670        11,026,748      6,572,502      1,665,112      1,121,468
      Interest                                             124,052         4,535,647      2,932,362      1,607,437        170,326
      Restructuring                                              -        11,020,000              -              -              -
                                                      ------------      ------------  ------------- -------------- --------------

            Total operating expenses                    50,861,247        73,410,310     30,482,234     9,675,086       4,739,072
                                                      -------------    -------------  ------------- --------------    -----------

            Loss before extraordinary item             (46,852,600)      (69,461,326)   (25,980,373)    (8,682,172)    (2,966,192)

EXTRAORDINARY ITEM:
      Gain on exchange of 9% convertible
      subordinated notes payable                               -                  -     8,876,685              -              -
                                                      ----------    ----------------  ------------ -------------- --------------

            Net loss                                   (46,852,600)      (69,461,326)   (17,103,688)    (8,682,172)    (2,966,192)

ACCRETION OF PREFERRED STOCK DIVIDENDS                           -                 -      2,689,048              -      1,042,052
                                                      ------------  ----------------    ----------- --------------   ------------

            Net loss applicable to                    $(46,852,600)     $(69,461,326)  $(19,792,736)   $(8,682,172)   $(4,008,244)
                                                      =============     =============  =============   ============   ============
               common stockholders

BASIC AND DILUTED NET LOSS PER COMMON SHARE:

            Loss per share before extraordinary item   $    (10.24)  $        (13.76) $       (2.19)        (1.72) $       (0.19)

            Extraordinary item                                   -                 -           0.75              -              -
                                                      ------------  ----------------  --------------   ------------  -------------

            Net loss per share                              (10.24)           (13.76)         (1.44)        (1.72)         (0.19)

            Accretion of preferred stock dividends               -                 -           (.23)             -         (0.07)
                                                      ------------  ---------------- --------------- -------------    -----------


            Net loss per share applicable to common
            stockholders                               $    (10.24)  $        (13.76) $       (1.67)        (1.72) $       (0.26)
                                                       =============     =============  =============   ============   ============

SHARES USED IN COMPUTING BASIC
AND DILUTED NET LOSS PER COMMON SHARE                    4,575,555         5,049,840     11,859,350      5,059,650     15,304,825
                                                       =============     =============  =============   ============   ============
</TABLE>


The  accompanying  notes are an integral  part of these  consolidated  financial
statements.
                                                                           F-4



<PAGE>





                         HYBRIDON, INC. AND SUBSIDIARIES
            CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (DEFICIT)
<TABLE>
<CAPTION>
                                                                                 Convertible      Series A Convertible
                                                                               Preferred Stock       Preferred Stock   Commom Stock

                                                                        Number of       $.01 Par   Number of  $.01 Par   Number of
                                                                          Shares         Value       Shares    Value       Shares
<S>                                                                    <C>          <C>               <C>      <C>     <C>
BALANCE, DECEMBER 31, 1995                                              3,196,435    $      31,965      -        -      $   368,733
     Issuance of common stock related to initial public offering,
         net of issuance costs of $5,268,756                                    -                -      -        -        1,150,000
     Conversion of convertible preferred stock to common stock         (3,196,435)         (31,965)     -        -        3,371,330
     Issuance of common stock related to the exercise of
          stock options                                                         -                -      -        -           57,740
     Issuance of common stock related to the exercise of
          warrants                                                              -                -      -        -           81,512
     Deferred compensation related to grants of stock options to
          nonemployees                                                          -                -      -        -                -
     Amortization of deferred compensation                                      -                -      -        -                -
     Net loss                                                                   -                -      -        -                -
                                                                    -------------    -------------    ---      ---    -------------
BALANCE, DECEMBER 31, 1996                                                      -                -      -        -        5,029,315
     Issuance of common stock related to the exercise of stock                  -                -      -        -           25,005
     Issuance of common stock related to the exercise of
          warrants                                                              -                -      -        -              330
     Issuance of common stock for services rendered                             -                -      -        -            5,000
     Deferred compensation related to grants of stock options to
          nonemployees                                                          -                -      -        -                -
     Amortization of deferred compensation                                      -                -      -        -                -
     Net loss                                                                   -                -      -        -                -
                                                                    -------------    -------------    ---      ---    -------------

BALANCE, DECEMBER 31, 1997                                                      -                -      -        -        5,059,650


</TABLE>
<TABLE>
<CAPTION>

                                                                    Common Stock
                                                                                      Additional
                                                                      $.001 Par        Paid-in       Accumulated         Deferred
                                                                        Value          Capital         Deficit         Compensation

<S>                                                                            <C>   <C>             <C>                    <C>
BALANCE, DECEMBER 31, 1995                                                     369   $ 114,755,394   $(102,341,175)            -
     Issuance of common stock related to initial public offering,
         net of issuance costs of $5,268,756                                 1,150      52,230,094               -             -
     Conversion of convertible preferred stock to common stock               3,371          28,594               -             -
     Issuance of common stock related to the exercise of
          stock options                                                         58       1,089,618               -             -
     Issuance of common stock related to the exercise of
          warrants                                                              81       3,176,660               -             -
     Deferred compensation related to grants of stock options to
          nonemployees                                                           -       1,967,116               -    (1,967,116)
     Amortization of deferred compensation                                       -               -               -       763,190
     Net loss                                                                    -               -     (46,852,600)            -
                                                                     -------------   -------------    ------------   -----------
BALANCE, DECEMBER 31, 1996                                                   5,029     173,247,476    (149,193,775)   (1,203,926)
     Issuance of common stock related to the exercise of stock                  26          86,300               -             -
     Issuance of common stock related to the exercise of
          warrants                                                               -           9,075               -             -
     Issuance of common stock for services rendered                              5         146,869               -             -
     Deferred compensation related to grants of stock options to
          nonemployees                                                           -         205,978               -      (205,978)
     Amortization of deferred compensation                                       -               -               -       316,067
     Net loss                                                                    -               -     (69,461,326)            -
                                                                     -------------   -------------    -------------  -----------

BALANCE, DECEMBER 31, 1997                                                   5,060     173,695,698    (218,655,101)   (1,093,837)

</TABLE>




                                                                    Total
                                                               Stockholders'
                                                              Equity (Deficit)

BALANCE, DECEMBER 31, 1995                                          12,446,553
     Issuance of common stock related to initial public offering,
         net of issuance costs of $5,268,756                        52,231,244
     Conversion of convertible preferred stock to common stock               -
     Issuance of common stock related to the exercise of
          stock options                                              1,089,676
     Issuance of common stock related to the exercise of
          warrants                                                   3,176,741
     Deferred compensation related to grants of stock options to
          nonemployees                                                       -
     Amortization of deferred compensation                             763,190
     Net loss                                                      (46,852,600)
                                                                   -----------
BALANCE, DECEMBER 31, 1996                                          22,854,804
     Issuance of common stock related to the exercise of stock          86,326
     Issuance of common stock related to the exercise of
          warrants                                                       9,075
     Issuance of common stock for services rendered                    146,874
     Deferred compensation related to grants of stock options to
          nonemployees                                                       -
     Amortization of deferred compensation                             316,067
     Net loss                                                      (69,461,326)
                                                                   -----------

BALANCE, DECEMBER 31, 1997                                         (46,048,180)


                                      F-5



<PAGE>




                         HYBRIDON, INC. AND SUBSIDIARIES
            CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (DEFICIT)
                                   (Continued)


<TABLE>
<CAPTION>

                                                                                 Convertible                    Series A Convertible
                                                                               Preferred Stock                     Preferred Stock

                                                                                Number of           $.01 Par           Number of
                                                                                 Shares             Value               Shares
<S>                                                                               <C>                 <C>            <C>
BALANCE, DECEMBER 31, 1997                                                           -                  -                   -
     Issuance of Series A convertible preferred stock and
         attached warrants in exchange for conversion of 9%
         convertible subordinated notes payable and accrued
         interest, net of issuance costs of $1,195,398                               -                  -             510,504
     Issuance of common stock and attached warrants in
         exchange for conversion of accounts payable and other
         obligations                                                                 -                  -                   -
     Issuance of Series A convertible preferred stock                                -                  -             114,285
     Issuance of common stock to Placement Agent                                     -                  -                   -
     Issuance of common stock and attached warrants in
         exchange for conversion of convertible notes payable, net                   -                  -                   -
         of issuance costs of $566,167
     Issuance of common stock and attached warrants, net of
         issuance costs of $1,069,970                                                -                  -                   -
     Issuance of common stock for services rendered                                  -                  -                   -
     Deferred compensation related to grants of
         stock options to nonemployees, net of terminations                          -                  -                   -
     Issuance of warrants in connection with notes payable                           -                  -                   -
     Accretion and issuance of Series A convertible preferred
         stock dividends                                                             -                  -              16,470
     Amortization of deferred compensation                                           -                  -                   -
     Net loss                                                                        -                  -                   -
                                                                        --------------     --------------      --------------     --
BALANCE, DECEMBER 31, 1998                                                           -                  -             641,259
     Accretion of Series A  convertible preferred
         stock dividends                                                             -                  -                   -
     Amortization of deferred compensation                                           -                  -                   -
     Net loss                                                                        -                  -                   -
                                                                        --------------     --------------      --------------     --
BALANCE, MARCH 31, 1999 (UNAUDITED)                                                  -     $            -             641,259
                                                                        ==============     ==============          ==========

</TABLE>





<TABLE>
<CAPTION>

                                                                         Series A Convertible
                                                                          Preferred Stock       Common Stock

                                                                                $.01 Par          Number of           $.001 Par
                                                                                Value              Shares              Value
<S>                                                                             <C>            <C>                     <C>
BALANCE, DECEMBER 31, 1997                                                          -          5,059,650               5,060
     Issuance of Series A convertible preferred stock and
         attached warrants in exchange for conversion of 9%
         convertible subordinated notes payable and accrued
         interest, net of issuance costs of $1,195,398                          5,105                  -                   -
     Issuance of common stock and attached warrants in
         exchange for conversion of accounts payable and other
         obligations                                                                -          3,217,154               3,217
     Issuance of Series A convertible preferred stock                           1,143                  -                   -
     Issuance of common stock to Placement Agent                                    -            597,699                 598
     Issuance of common stock and attached warrants in
         exchange for conversion of convertible notes payable, net                  -          3,157,322               3,157
         of issuance costs of $566,167
     Issuance of common stock and attached warrants, net of
         issuance costs of $1,069,970                                               -          3,223,000               3,223
     Issuance of common stock for services rendered                                 -             50,000                  50
     Deferred compensation related to grants of
         stock options to nonemployees, net of terminations                         -                  -                   -
     Issuance of warrants in connection with notes payable                          -                  -                   -
     Accretion and issuance of Series A convertible preferred
         stock dividends                                                          165                  -                   -
     Amortization of deferred compensation                                          -                  -                   -
     Net loss                                                                       -                  -                   -
                                                                       --------------     --------------      --------------
BALANCE, DECEMBER 31, 1998                                                      6,413         15,304,825              15,305
     Accretion of Series A  convertible preferred
         stock dividends                                                            -                  -                   -
     Amortization of deferred compensation                                          -                  -                   -
     Net loss                                                                       -                  -                   -
                                                                       --------------     --------------      --------------
BALANCE, MARCH 31, 1999 (UNAUDITED)                                       $     6,413         15,304,825   $          15,305
                                                                          ===========         ==========  ==================

</TABLE>


<TABLE>
<CAPTION>


                                                                               Additional         Accumulated         Deferred
                                                                             Paid-in Capital        Deficit         Compensation
<S>                                                                         <C>                <C>                  <C>
BALANCE, DECEMBER 31, 1997                                                  173,695,698        (218,655,101)        (1,093,837)
     Issuance of Series A convertible preferred stock and
         attached warrants in exchange for conversion of 9%
         convertible subordinated notes payable and accrued
         interest, net of issuance costs of $1,195,398                       38,729,489                   -                  -
     Issuance of common stock and attached warrants in
         exchange for conversion of accounts payable and other
         obligations                                                          5,931,341                   -                  -
     Issuance of Series A convertible preferred stock                         7,998,817                   -                  -
     Issuance of common stock to Placement Agent                              1,194,800                   -                  -
     Issuance of common stock and attached warrants in
         exchange for conversion of convertible notes payable, net            4,230,676                   -                  -
         of issuance costs of $566,167
     Issuance of common stock and attached warrants, net of
         issuance costs of $1,069,970                                         6,873,453                   -                  -
     Issuance of common stock for services rendered                              93,700                   -                  -
     Deferred compensation related to grants of
         stock options to nonemployees, net of terminations                     109,734                   -           (109,734)
     Issuance of warrants in connection with notes payable                       85,433                   -                  -
     Accretion and issuance of Series A convertible preferred
         stock dividends                                                      2,688,883          (2,689,048)                 -
     Amortization of deferred compensation                                            -                   -            246,444
     Net loss                                                                          -        (17,103,688)                 -
                                                                       -----------------     ---------------    --------------
BALANCE, DECEMBER 31, 1998                                                  241,632,024        (238,447,837)          (957,127)
     Accretion of Series A  convertible preferred
         stock dividends                                                      1,042,052          (1,042,052)                 -
     Amortization of deferred compensation                                             -                  -             61,611
     Net loss                                                                             -      (2,966,192)                 -
                                                                       --------------------   --------------    ---------------
BALANCE, MARCH 31, 1999 (UNAUDITED)                                     $  (242,674,076)     $ (242,456,081)    $     (895,516)
                                                                        ================     ===============    ===============
</TABLE>





<TABLE>
<CAPTION>

                                                                             Total
                                                                           Stockholders'
                                                                           Equity (Deficit)
<S>                                                                         <C>
BALANCE, DECEMBER 31, 1997                                                  (46,048,180)
     Issuance of Series A convertible preferred stock and
         attached warrants in exchange for conversion of 9%
         convertible subordinated notes payable and accrued
         interest, net of issuance costs of $1,195,398                       38,734,594
     Issuance of common stock and attached warrants in
         exchange for conversion of accounts payable and other
         obligations                                                          5,934,558
     Issuance of Series A convertible preferred stock                         7,999,960
     Issuance of common stock to Placement Agent                              1,195,398
     Issuance of common stock and attached warrants in
         exchange for conversion of convertible notes payable, net            4,233,833
         of issuance costs of $566,167
     Issuance of common stock and attached warrants, net of
         issuance costs of $1,069,970                                         6,876,676
     Issuance of common stock for services rendered                              93,750
     Deferred compensation related to grants of
         stock options to nonemployees, net of terminations                           -
     Issuance of warrants in connection with notes payable                       85,433
     Accretion and issuance of Series A convertible preferred
         stock dividends                                                              -
     Amortization of deferred compensation                                      246,444
     Net loss                                                               (17,103,688)
                                                                         ----------------
BALANCE, DECEMBER 31, 1998                                                    2,248,778
     Accretion of Series A  convertible preferred
         stock dividends                                                              -
     Amortization of deferred compensation                                       61,611
     Net loss                                                                (2,966,192)
                                                                         ---------------
BALANCE, MARCH 31, 1999 (UNAUDITED)                                      $     (655,803)
                                                                         ===============
</TABLE>








The  accompanying  notes are an integral  part of these  consolidated
statements.



                                       F-6




<PAGE>


                         HYBRIDON, INC. AND SUBSIDIARIES

                      CONSOLIDATED STATEMENTS OF CASH FLOWS
<TABLE>
<CAPTION>

                                                                                         Years Ended December 31,

                                                                               1996                 1997               1998
<S>                                                                           <C>                  <C>                <C>
CASH FLOWS FROM OPERATING ACTIVITIES:
     Net loss                                                                 $(46,852,600)        $(69,461,326)      $(17,103,688)
     Adjustments to reconcile net loss to net cash used in operating activities-
         Extraordinary gain on exchange of 9% convertible
         subordinated notes payable                                                      -                    -         (8,876,685)
         Depreciation and amortization                                           2,393,751            4,488,719          4,057,286
         Loss on disposal of fixed assets                                                -                    -                  -
         Issuance of common stock for services rendered                                  -              146,874             93,750
         Amortization of deferred compensation                                     763,190              316,067            246,444
         Amortization of deferred financing costs                                        -              479,737            160,813
         Noncash portion of restructuring charge                                         -            1,255,000                  -
         Changes in assets and liabilities-
              Accounts receivable                                                 (573,896)              44,194           (645,739)
              Prepaid expenses and other current assets                           (593,797)             539,499            894,998
              Note receivable from officer                                          (9,845)              70,728            (11,400)
              Accounts payable                                                   2,010,981            3,987,398         (3,059,002)
              Accrued expenses                                                     736,141            7,071,532          1,565,806
              Deferred revenue                                                           -              (86,250)                 -
              Amounts payable to related parties                                   (12,500)                   -                  -
                                                                               -----------         ------------      -------------
                  Net cash used in operating activities                        (42,138,575)         (51,147,828)       (22,677,417)
                                                                               -----------         ------------      -------------
CASH FLOWS FROM INVESTING ACTIVITIES:
     (Increase) decrease in short-term investments                              (3,785,146)           3,785,146                  -
     Purchases of property and equipment                                        (8,902,989)          (7,509,755)          (471,949)
     Proceeds from sale of property and equipment                                        -                    -            714,400
     (Investment in) sale of real estate partnership                            (3,751,552)                   -          5,450,000
                                                                               ------------        ------------      -------------
                  Net cash (used in) provided by investing activities          (16,439,687)          (3,724,609)         5,692,451
                                                                               -----------         ------------      -------------
CASH FLOWS FROM FINANCING ACTIVITIES:
     Proceeds from issuance of Series A convertible preferred

              stock                                                                      -                    -          7,999,960
     Proceeds from issuance of common stock related to stock options

              and restricted stock grants                                        1,089,676               86,326                  -
     Net proceeds from issuance of common stock                                 52,231,244                    -          6,876,676
     Proceeds from notes payable                                                 7,500,000                    -          6,000,000
     Proceeds from issuance of convertible promissory notes payable                      -           50,000,000          4,233,833
     Proceeds from issuance of common stock related to stock

              warrants                                                           3,176,741                9,075                  -
     Proceeds from sale/leaseback of fixed assets                                1,722,333            1,205,502                  -
     Payments on long-term debt                                                   (446,163)          (1,564,268)        (7,296,646)
     Decrease (increase) in deferred financing costs                               251,921           (2,820,790)          (400,000)
     Decrease (increase) in restricted cash and other assets                       401,990           (2,474,948)         2,976,823
                                                                               -----------         ------------      -------------
                  Net cash provided by (used in) financing activities           65,927,742           44,440,897         20,390,646
                                                                               -----------         ------------      -------------
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS                             7,349,480          (10,431,540)         3,405,680
CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR                                     5,284,262           12,633,742          2,202,202
                                                                               -----------         ------------      -------------
CASH AND CASH EQUIVALENTS, END OF YEAR                                         $12,633,742         $  2,202,202      $   5,607,882
                                                                               ===========         ============      =============




                                                                                       Three Months Ended March 31,

                                                                                          1998                  1999
CASH FLOWS FROM OPERATING ACTIVITIES:                                                                        (Unaudited)
     Net loss                                                                            $(8,682,172)        $(2,966,192)
     Adjustments to reconcile net loss to net cash used in operating activities-
         Extraordinary gain on exchange of 9% convertible
         subordinated notes payable                                                                 -                   -
         Depreciation and amortization                                                      1,130,540             690,831
         Loss on disposal of fixed assets                                                     359,424                   -
         Issuance of common stock for services rendered                                             -                   -
         Amortization of deferred compensation                                                 54,348              61,611
         Amortization of deferred financing costs                                             217,021              26,984
         Noncash portion of restructuring charge                                                    -                   -
         Changes in assets and liabilities-
              Accounts receivable                                                               7,815            (103,051)
              Prepaid expenses and other current assets                                       298,956               5,406
              Note receivable from officer                                                     (2,850)             (2,850)
              Accounts payable                                                               (479,432)           (529,180)
              Accrued expenses                                                                745,990            (315,550)
              Deferred revenue                                                                      -                   -
              Amounts payable to related parties                                                    -                   -
                                                                                      ---------------     ---------------
                  Net cash used in operating activities                                    (6,350,360)         (3,131,991)
                                                                                      ---------------     ---------------
CASH FLOWS FROM INVESTING ACTIVITIES:
     (Increase) decrease in short-term investments                                                  -                   -
     Purchases of property and equipment                                                            -                   -
     Proceeds from sale of property and equipment                                             400,000               2,180
     (Investment in) sale of real estate partnership                                               -                    -
                                                                                              -------               -----
                                                                                              400,000               2,180
                                                                                              -------               -----
CASH FLOWS FROM FINANCING ACTIVITIES:
     Proceeds from issuance of Series A convertible preferred

              stock                                                                                 -                   -
     Proceeds from issuance of common stock related to stock options

              and restricted stock grants                                                           -                   -
     Net proceeds from issuance of common stock                                                     -                   -
     Proceeds from notes payable                                                                    -                   -
     Proceeds from issuance of convertible promissory notes payable                         4,233,833                   -
     Proceeds from issuance of common stock related to stock

              warrants                                                                              -                   -
     Proceeds from sale/leaseback of fixed assets                                                   -                   -
     Payments on long-term debt                                                            (2,204,315)            (16,887)
     Decrease (increase) in deferred financing costs                                                -                   -
     Decrease (increase) in restricted cash and other assets                                2,157,854                   -
                                                                                           ----------          ----------
                  Net cash provided by (used in) financing activities                       4,187,372             (16,887)
                                                                                           ----------          ----------
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS                                       (1,762,988)         (3,146,698)
CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR                                                2,202,202           5,607,882
                                                                                           ----------          ----------
CASH AND CASH EQUIVALENTS, END OF YEAR                                                     $  439,214          $2,461,184
                                                                                           ==========          ==========


</TABLE>



The  accompanying  notes are an integral  part of these  consolidated  financial
statements.
                                                                           F-7



<PAGE>



                         HYBRIDON, INC. AND SUBSIDIARIES

                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS

                (Including Data Applicable to Unaudited Periods)


(1)  ORGANIZATION

     Hybridon,  Inc. (the Company) was  incorporated in the State of Delaware on
     May 25, 1989.  The Company is engaged in the discovery and  development  of
     novel genetic medicines based primarily on antisense technology.

     Since inception,  the Company has devoted  substantially all of its efforts
     toward product research and development,  its custom contract manufacturing
     business  (Hybridon   Specialty  Products  or  HSP)  and  raising  capital.
     Management  anticipates  that  substantially  all future  revenues  will be
     derived  from  the sale of  proprietary  biopharmaceutical  products  under
     development  or  to  be  developed  in  the  future,  and  custom  contract
     manufacturing  of synthetic DNA products and reagent  products (by HSP), as
     well as from  research  and  development  revenues  and fees and  royalties
     derived from licensing of the Company's technology.  Accordingly,  although
     the  Company  has  begun to  generate  revenues  from its  custom  contract
     manufacturing  business,  the Company is  dependent  on the  proceeds  from
     possible  future  sales of debt and  equity  securities  and  research  and
     development  collaborations  in order to fund future  operations.  There is
     substantial doubt concerning its ability to continue as a going concern. As
     of  December  31,  1998,  the  Company  had cash and  cash  equivalents  of
     approximately  $5.6  million.  The Company  expects such  resources to fund
     operations through May 1999. The consolidated  financial  statements do not
     include  any  adjustments  that  might  result  from  the  outcome  of this
     uncertainty.

     The Company is  currently  seeking  debt or equity  financing  in an amount
     sufficient  to  support  its  operations  through  the end of 1999,  and in
     connection  therewith,  is in  negotiations  with several parties to obtain
     such financing.  If the Company is unable to obtain this sufficient  amount
     of  additional  funding  in May 1999,  it will be forced to  terminate  its
     operations or seek relief under applicable bankruptcy law by the end of May
     1999.

     See Note 22 for first-quarter 1999 update.

     On December 3, 1997, the Company was delisted from the Nasdaq Stock Market,
     Inc.  (NASDAQ) because the Company was not in compliance with the continued
     listing  requirements  of  the  NASDAQ  National  Market.  The  Company  is
     currently trading on the NASD OTC as a result of the delisting.

(2)  SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

     (a)  Management Estimates and Uncertainties

          The  preparation of financial  statements in conformity with generally
          accepted  accounting  principles requires management to make estimates
          and  assumptions  that  affect  the  reported  amounts  of assets  and
          liabilities and disclosure of contingent assets and liabilities at the
          date of the financial  statements and the reported amounts of revenues
          and expenses during the reporting period.  Actual results could differ
          from those estimates.


                                       F-8



<PAGE>


                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENT
                (Including Data Applicable To Unaudited Periods)
                                   (continued)

          The Company is subject to a number of risks and uncertainties  similar
          to those of other companies of the same size within the  biotechnology
          industry,  such as uncertainty  with clinical  trials,  uncertainty of
          additional funding and history of operating losses.

     (b)  Principles of Consolidation

          The accompanying consolidated financial statements include the results
          of the Company and its subsidiaries,  Hybridon S.A. (Europe), a French
          corporation,  and Hybridon  Canada,  Inc. (an inactive  majority-owned
          subsidiary).  The consolidated  financial  statements also reflect the
          Company's 30% interest in MethylGene,  Inc.  (MethylGene),  a Canadian
          corporation  which is accounted  for under the equity method (see Note
          14). All material  intercompany  balances and  transactions  have been
          eliminated in consolidation.

     (c)  Cash Equivalents

          The Company considers all highly liquid investments with maturities of
          three months or less when purchased to be cash  equivalents.  Cash and
          cash  equivalents  and  restricted  cash at December 31, 1997 and 1998
          consisted of the following (at amortized cost, which approximates fair
          market value):
<TABLE>
<CAPTION>

                                                                       1997                       1998
<S>                                                                <C>                        <C>
       Cash and cash equivalents-
                Cash and money market funds                        $1,702,272                 $3,865,365
                Corporate bond                                        499,930                  1,742,517
                                                                   ----------                 ----------
                Total cash and cash equivalents                    $2,202,202                 $5,607,882
                                                                   ==========                 ==========

       Restricted cash-
                Note payable to bank (Note 7(a))                   $1,758,542                 $        -
                Foreign bank account (Note 6)                       1,034,618                          -
                Capital lease obligations (Note 7(d))                 257,822                          -
                                                                   ----------                 ----------
                                                                   $3,050,982                 $       -
                                                                   ==========                 ==========
</TABLE>


     (d)  Depreciation and Amortization

          Depreciation  and  amortization  are computed using the  straight-line
          method based on the  estimated  useful lives of the related  assets as
          follows:

                Asset Classification                       Estimated
                                                           Useful Life

          Leasehold improvements                          Life of lease
          Laboratory equipment and other                    3-5 years




                                       F-9



<PAGE>


                         HYBRIDON, INC. AND SUBSIDIARIES

                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)

     (e)  Accrued Expenses

          At  December  31,  1997 and  1998,  accrued  expenses  consist  of the
          following:
<TABLE>
<CAPTION>

                                                                              1997                       1998

                     <S>                                                  <C>                          <C>
                      Restructuring (Note 3)                               $8,316,148                   $469,485
                      Interest                                              1,125,000                     29,385
                      Payroll and related costs                               742,452                  1,151,742
                      Outside research and clinical costs                   1,231,818                    797,593
                      Professional fees                                       150,000                    149,957
                      Contingent stock (Notes 7(b) and 15(c))                       -                  1,000,000
                      Other                                                   351,880                    470,517
                                                                          -----------                 ----------
                                                                          $11,917,298                 $4,068,679
                                                                          ===========                 ==========
</TABLE>


     (f)  Reclassifications

          Certain amounts in the prior periods consolidated financial statements
          have  been   reclassified   to  conform  with  the  current   period's
          presentation.

     (g)  Revenue Recognition

          The Company has recorded  revenue  under the  consulting  and research
          agreements  discussed in Notes 8, 9 and 14.  Revenue is  recognized as
          earned on a straight-line basis over the term of the agreement,  which
          approximates  when work is performed and costs are incurred.  Revenues
          from product and service  sales are  recognized  when the products are
          shipped or the services are performed. Product revenue during 1997 and
          1998   represents   revenues   from  the   sale  of   oligonucleotides
          manufactured on a custom contract basis by HSP.

     (h)  Research and Development Expenses

          The Company charges research and development expenses to operations as
          incurred.

     (i)  Patent Costs

          The Company charges patent expenses to operations as incurred.


                                      F-10



<PAGE>


                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)

     (j)  Comprehensive Loss

          The Company  applies  SFAS No. 130,  Reporting  Comprehensive  Income.
          Comprehensive  loss is  defined  as the change in equity of a business
          enterprise  during a period  from  transactions  and other  events and
          circumstances from nonowner sources. The Company's  comprehensive loss
          is the same as the reported net loss for all periods presented.

     (k)  Net Loss per Common Share

          The Company  applies SFAS No 128,  Earnings per Share.  Under SFAS No.
          128,  basic net loss per common  share is computed  using the weighted
          average  number  of  shares of common  stock  outstanding  during  the
          period.  Diluted  net loss per  common  share is the same as basic net
          loss per common share as the effects of the Company's potential common
          stock  equivalents are  antidilutive.  Antidilutive  securities  which
          consist of stock options, warrants and convertible preferred stock (on
          an  as-converted  basis) that are not included in diluted net loss per
          common share were 2,595,496,  2,404,561 and 27,774,883 for 1996, 1997,
          and 1998, respectively.

     (l)  Segment Reporting

          The Company  applies SFAS No. 131,  Disclosures  About  Segments of an
          Enterprise and Related Information. SFAS No. 131 establishes standards
          for  reporting  information  regarding  operating  segments  in annual
          financial  statements  and  requires  selected  information  for those
          segments  to be  presented  in  interim  financial  reports  issued to
          stockholders.  SFAS No. 131 also  establishes  standards  for  related
          disclosures about products and services and geographic areas. To date,
          the Company  has viewed its  operations  and  manages its  business as
          principally  one  operating  segment.   As  a  result,  the  financial
          information disclosed herein, represents all of the material financial
          information related to the Company's principal operating segment.  All
          of the  Company's  revenues  are  generated  in the United  States and
          substantially all assets are located in the United States.

(3)  RESTRUCTURING

     Beginning in July 1997,  the Company  implemented a  restructuring  plan to
     reduce  expenditures  on a phased  basis in an effort to conserve  its cash
     resources.  As part of this restructuring  plan, in addition to terminating
     the  clinical  development  of  GEM  91,  the  Company's  first  generation
     antisense  drug for the  treatment of AIDS and HIV  infection,  the Company
     reduced or  suspended  programs  unrelated to its core  advanced  chemistry
     antisense drug research and  development  programs.  In connection with the
     reduction in programs,  the Company has accrued termination fees related to
     research contracts and has written off assets related to programs that have
     been  suspended  or  canceled.  As part of the  restructuring,  all outside
     testing,   public  relations,   travel  and  entertainment  and  consulting
     arrangements   were   reviewed  and  where   appropriate   the  terms  were
     renegotiated,  contracts cancelled or the terms were significantly reduced.
     As a result of the implementation of these changes,  the Company terminated
     the employment of 84 employees at its Cambridge and Milford, Massachusetts,
     facilities  in 1997  and  closed  its  operations  in  Paris,  France,  and
     terminated 11 employees at that location.


                                      F-11



<PAGE>


                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)

     In connection  with the  restructuring,  the Company entered into different
     subleasing  arrangements.  During 1997, the Company  subleased a portion of
     each of its facilities in Cambridge, Massachusetts (including a substantial
     portion  of its former  headquarters  located  at 620  Memorial  Drive (the
     Cambridge  Headquarters)).  The Company incurred expenses relating to these
     subleases for broker fees and renovation expenses incurred in preparing the
     Cambridge  Headquarters space for the new tenant. In addition,  the Company
     accrued the estimated  lease loss of subleasing the Cambridge  Headquarters
     which were vacated  during 1998.  The Company also  subleased its office in
     Paris, France, and accrued the estimated lease loss.

     The following are the significant  components of the $11,020,000 charge for
     restructuring (in thousands):
<TABLE>
<CAPTION>

                                                                                                         To be Paid
                                                                                                            as of
                                                     Restructuring     Non-Cash             Cash         December 31,
                                                         Charge        Portion           Disbursed           1998

<S>                                                     <C>          <C>             <C>             <C>
Estimated loss on facility leases                       $    6,372   $   5,976       $       356     $      40
Employee severance, benefits and
    related costs                                            2,738           -             2,548           190
Write-down of assets to net realizable
    value                                                      946         946                 -             -
Termination costs of certain
    research programs                                          964         672                53           239
                                                        ----------    --------       ----------      --------
                                                        $   11,020    $  7,594        $    2,957      $    469
                                                        ==========    ========        ==========      ========
</TABLE>


     The Company disbursed cash totaling approximately $1,453,000 and $1,504,000
     in 1997 and 1998,  respectively,  with  respect to the  restructuring.  The
     remaining  accrued  amount of  approximately  $469,000  will be paid during
     1999.

(4)  INVESTMENT IN REAL ESTATE PARTNERSHIP

     Under the terms of the lease for the Cambridge  Headquarters (the Cambridge
     Lease),  the Company accounted for $5,450,000 of its payments for a portion
     of the costs of construction of the leased premises as contributions to the
     capital of the  Cambridge  landlord in exchange  for a limited  partnership
     interest in the Cambridge  landlord (the Partnership  Interest).  Under the
     terms of the Partnership Interest,  the Company exercised its right to sell
     back the  Partnership  Interest and received  payment of the  $5,450,000 in
     1998.

(5)  NOTE RECEIVABLE FROM OFFICER

     At  December  31,  1997 and 1998 the  Company  has a note  receivable  from
     officer,   including   accrued   interest,   of  $247,250   and   $258,650,
     respectively.  The note has an interest  rate of 6.0% per annum and matures
     in April 2001.

(6)  RESTRICTED CASH - BVH

     In November 1997, the Company was notified by Bank Fur Vermogensanlagen Und
     Handel AG (BVH) that the Federal Banking  Supervisory Office in Germany had
     imposed a moratorium on

                                      F-12



<PAGE>


                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)

     BVH and had closed BVH for business.  Accordingly,  the Company  classified
     its deposit with BVH as  restricted  cash.  The Company sold the deposit to
     the Cambridge  Landlord,  an affiliate of certain directors of the Company,
     and recovered the full amount in 1998.

(7)  LONG-TERM DEBT AND CAPITAL LEASE OBLIGATIONS

     Future  minimum  principal   payments  due  under  various  notes  payable,
     excluding the 9% convertible subordinated notes (the 9% Notes) due April 1,
     2004, are as follows at December 31, 1998:

              December 31,                                             Amount

              1999                                                 $ 6,070,951
              2000                                                      80,746
              2001                                                      91,892
              2002                                                     104,576
              2003                                                     119,010
              Thereafter                                                76,870
                                                                   -----------

                       Total long-term debt obligations              6,544,045

              Less--Current portion                                   6,070,951

                                                                   $   473,094
                                                                   ===========


     (a)  Note Payable to a Bank

          In December 1996, the Company entered into a five-year $7,500,000 note
          payable  to a bank.  In  November  1998,  the  outstanding  balance of
          approximately  $2,895,000 was purchased from the bank by Forum Capital
          Markets,  LLC  (Forum)  and certain  investors  associated  with Pecks
          Management Partners Ltd. (Pecks)  (collectively,  the Lenders),  which
          are affiliates of two members of the Company's Board of Directors.

     (b)  Note Payable to Lenders

          In connection  with the purchase by the Lenders of the note payable to
          the bank, the Lenders lent an additional  $3,200,000 so as to increase
          the outstanding principal amount of the note to $6,000,000.  The terms
          of the note  payable  were  amended as follows:  (i) the  maturity was
          extended to November 30, 2003; (ii) the interest rate was decreased to
          8%; (iii) interest is payable  monthly in arrears,  with the principal
          due in  full at  maturity  of the  loan;  (iv)  the  note  payable  is
          convertible,  at the Lenders' option, in whole or in part, into shares
          of common  stock at a conversion  price equal to $2.40 per share;  (v)
          the  note  includes  a  minimum  liquidity,  as  defined  covenant  of
          $2,000,000;  and (vi) the note payable may not be prepaid, in whole or
          in part, at any time prior to December 1, 2000. On March 30, 1999, the
          Company received a waiver for noncompliance  with the minimum tangible
          net worth  covenant  effective  as of December  31, 1998 and March 31,
          1999.  On April 15,  1999,  the  Company  also  received  a waiver for
          non-compliance  with the
                                      F-13



<PAGE>


                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)

     minimum liquidity  covenant effective as of April 15, 1999. The Company has
     classified the outstanding  balance of $6,000,000 at December 31, 1998 as a
     current liability in the accompanying consolidated balance sheet as it does
     not currently have the financing to remain in compliance with the financial
     covenants.  In connection  with the purchase of the note payable,  Forum is
     entitled to receive  $400,000 as a fee,  which Forum has agreed to reinvest
     by purchasing common stock or preferred stock, both with attached warrants.
     The Company has recorded the $400,000 as a deferred  financing cost,  which
     will be  amortized  to  interest  expense  over the term of the note and an
     accrued expense for the issuance of common stock or preferred  stock,  both
     with attached  warrants,  which will occur in 1999.  In addition,  Forum is
     entitled to receive warrants to purchase $400,000 of shares of common stock
     of the Company at the per share valuation of the next  financing,  or $3.00
     per share if the  financing is not  completed  by May 1, 1999.  The Company
     determined  the  value  of  the  warrants  to  be  $85,433,  by  using  the
     Black-Scholes  option pricing model.  The Company has recorded this $85,433
     as a deferred  financing cost,  which will be amortized to interest expense
     over the term of the note.

     (c)  Note Payable to Landlord

          In December 1994, the Company issued a $750,000 promissory note to its
          landlord  to fund  specific  construction  costs  associated  with the
          development of its manufacturing plant in Milford, Massachusetts.  The
          promissory  note bears  interest at 13% per annum and is to be paid in
          equal  monthly   installments  of  principal  and  interest  over  the
          remainder of the 10-year lease term.

     (d)  Capital Lease Obligations

          The Company had entered into  various  capital  leases for  equipment.
          During 1998, the Company settled its capital lease obligations in full
          through the issuance of common stock and warrants (see Note 15 (c)).

     (e)  9% Convertible Subordinated Notes Payable

          On April 2, 1997,  the  Company  issued  $50,000,000  of the 9% Notes.
          Under the  terms of the 9% Notes,  the  Company  must make  semiannual
          interest  payments on the  outstanding  principal  balance through the
          maturity date of April 1, 2004. If the 9% Notes are converted prior to
          April 1,  2000,  the  noteholders  are  entitled  to  receive  accrued
          interest from the date of the most recent interest payment through the
          conversion date. The 9% Notes are convertible at any time prior to the
          maturity  date at a  conversion  price equal to  $35.0625,  subject to
          adjustment under certain circumstances, as defined.

          Beginning  April 1, 2000,  the  Company may redeem the 9% Notes at its
          option for a 4.5% premium over the original  issuance  price  provided
          that from  April 1, 2000 to March  31,  2001,  the 9% Notes may not be
          redeemed  unless  the  closing  price of the  common  stock  equals or
          exceeds 150% of the  conversion  price for a period of at least 20 out
          of 30 consecutive trading days and the 9% Notes are redeemed within 60
          days after such  trading  period.  The premium  decreases by 1.5% each
          year through March 31, 2003.  Upon a


                                      F-14



<PAGE>

                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)

     change of control of the Company, as defined,  the Company will be required
     to offer to repurchase the 9% Notes at 150% of the original issuance price.
     On February 6, 1998, the Company commenced an exchange offer to the holders
     of the 9% Notes to exchange the 9% Notes for Series A convertible preferred
     stock and warrants.  On May 5, 1998,  noteholders  holding  $48,694,000  of
     principal and  $2,361,850 of accrued  interest  tendered such principal and
     accrued  interest to the Company for 510,505 shares of Series A convertible
     preferred stock and warrants to purchase  3,002,958  shares of common stock
     with an exercise price of $4.25 per share.  In accordance with SFAS No. 15,
     Accounting by Debtors and Creditors for Troubled Debt  Restructurings,  the
     Company  recorded  an  extraordinary  gain  of  $8,876,685  related  to the
     exchange.  The  extraordinary  gain  represents the difference  between the
     carrying value of the 9% Notes plus accrued  interest,  less  $2,249,173 of
     deferred  financing  costs  written off, and the fair value of the Series A
     convertible  preferred stock, as determined by the per share sales price of
     Series A convertible  preferred  stock sold in the 1998 Unit Financing (see
     Note 15(c)), and warrants to purchase common stock issued by the Company.

(8)      G.D. SEARLE & CO. AGREEMENT

          In January 1996, the Company and G.D.  Searle & Co.  (Searle)  entered
          into  a   collaboration   relating  to  research  and  development  of
          therapeutic  antisense  compounds.   According  to  the  collaboration
          agreement,  as modified in April  1998,  targets can be selected  from
          those  in  the   fields  of   cancer,   cardiovascular   disease   and
          inflammation/immunomodulation (the Searle Field).

          Pursuant to the collaboration, the parties are conducting research and
          development  relating to a compound directed at MDM2. In this project,
          Searle is funding  certain  research  and  development  efforts by the
          Company, and both Searle and the Company have committed certain of its
          own personnel to the collaboration.  The initial phase of research and
          development  activities  will be conducted  through the earlier of (i)
          the  achievement  of certain  milestones,  and (ii)  January 31, 2000,
          subject to early  termination  by Searle.  The  parties may extend the
          initial  collaboration by mutual agreement,  including agreement as to
          additional research funding by Searle.

          In  addition,  under  the  collaboration,  Searle  has  the  right  to
          designate up to six additional  molecular  targets in the Searle Field
          (the Additional  Targets) on terms  substantially  consistent with the
          terms of the collaboration applicable to the initial molecular target.
          This  right is  exercisable  by  Searle  with  respect  to each of the
          Additional  Targets  upon the  payment by Searle of  certain  research
          payments  (beyond  the  project-specific   payments  relating  to  the
          particular  Additional  Target) and the purchase of additional  common
          stock from the Company by Searle (at the then fair market value).  The
          aggregate  amount to be paid by Searle for such research  payments and
          equity investment in order to designate each of the Additional Targets
          is  $10,000,000  per  Additional  Target.  In the  event  that  Searle
          designates all of the Additional  Targets,  the aggregate amount to be
          paid by Searle for  research  payments  will be  $24,000,000,  and the
          aggregate  amount to be paid by Searle  in equity  investment  will be
          $36,000,000.  If  Searle  has  not  designated  all of the  Additional
          Targets by the time the  initial  molecular  target  reaches a certain
          stage of preclinical development,  Searle will be required to purchase
          an
                                      F-15



<PAGE>

                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)

     additional  $10,000,000  of common stock (at the then fair market value) in
     order to maintain its right to designate any of the Additional Targets. The
     payment for any such  common  stock will be  creditable  against the equity
     investment portion of the payments to be made by Searle with respect to the
     designation  of any of the  Additional  Targets  that  Searle  has  not yet
     designated.  Searle has  exclusive  rights to  commercialize  any  products
     resulting  from the  collaboration.  If Searle  elects to  commercialize  a
     product, Searle will fund and perform preclinical tests and clinical trials
     of the product  candidate and will be responsible for regulatory  approvals
     for and  marketing  of the  product.  The  Company  has  agreed to  perform
     research and development  work exclusively  with Searle.  In addition,  for
     each product candidate,  the Company will be entitled to milestone payments
     from Searle totaling up to an aggregate of $10,000,000 upon the achievement
     of certain  development  benchmarks.  The Company  also will be entitled to
     royalties  from net sales of  products  resulting  from the  collaboration.
     Subject to  satisfying  certain  conditions  relating to its  manufacturing
     capacities and capabilities,  the Company will retain manufacturing rights,
     and Searle will be required to purchase its  requirements  of products from
     the Company on an  exclusive  basis at specified  prices.  Upon a change in
     control  of the  Company,  Searle  would  have the right to  terminate  the
     Company's  manufacturing  rights,  although  the royalty  payable  would be
     increased in such event.

          In the event that Searle  designates all of the Additional  Targets or
          if  Hybridon  fails to satisfy  certain  requirements  relating to its
          manufacturing capacities and capabilities,  Searle will have the right
          to require  Hybridon to form a joint venture with Searle,  as defined.
          The  Company  and  Searle  would  each own 50% of the  joint  venture,
          although  Searle's  ownership  interest  in the  joint  venture  would
          increase  based  upon a formula to up to a maximum of 75% if the joint
          venture is established in certain instances  relating to the Company's
          failure to satisfy certain requirements  relating to its manufacturing
          capacities and capabilities.

          During 1996, 1997 and 1998, the Company earned $400,000,  $600,000 and
          $600,000,  respectively,  in research and  development  revenues  from
          Searle. Under the collaboration,  Searle also purchased 200,000 shares
          of common  stock in the  Company at the  offering  price of $50.00 per
          share.

(9)       F. HOFFMANN-LA ROCHE LTD. (ROCHE) COLLABORATION

          In December 1992,  the Company and Roche entered into a  collaboration
          involving the application of the Company's  antisense  oligonucleotide
          chemistry  to develop  compounds  for the  treatment  of  hepatitis B,
          hepatitis C and human  papilloma  virus.  On September 3, 1997,  Roche
          notified  the  Company  that  it had  decided  not to  pursue  further
          collaboration  with the Company and was terminating the  collaboration
          effective February 28, 1998.

          The Company  has  recorded  $1,019,389  and  $345,000 of research  and
          development  revenue related to this  collaboration  in 1996 and 1997,
          respectively.  Due  to  the  termination  of  the  collaboration,   as
          discussed  above,  the Company  recognized  no revenue with respect to
          this collaboration in 1998.


                                      F-16



<PAGE>

                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)

(10)      MEDTRONIC, INC. COLLABORATIVE STUDY AGREEMENT

          In May 1994, the Company and Medtronic,  Inc. (Medtronic) entered into
          a collaborative  study agreement (the Medtronic  Agreement)  involving
          the   development   of  antisense   compounds  for  the  treatment  of
          Alzheimer's  disease  and a  drug  delivery  system  to  deliver  such
          compounds into the central nervous system. The agreement provides that
          the Company is responsible for the development of, and hold all rights
          to, any drug developed pursuant to this  collaboration,  and Medtronic
          is  responsible  for the  development  of, and hold all rights to, any
          delivery system developed pursuant to this collaboration.  The parties
          may  extend  this   collaboration   by  mutual   agreement   to  other
          neurodegenerative  disease  targets.  The  Company  is  not  currently
          conducting any activities under this collaboration.

(11)      LICENSING AGREEMENT

          The Company has entered into a licensing  agreement with the Worcester
          Foundation for Biomedical  Research,  Inc.,  which has merged with the
          University of  Massachusetts  Medical Center,  under which the Company
          has  received   exclusive  licenses  to  certain  patents  and  patent
          applications.  The Company is required to make royalty  payments based
          on future sales of products  employing the technology or falling under
          claims of a patent,  as well as a specified  percentage  of sublicense
          income received related to the licensed technology.  Additionally, the
          Company is required to pay an annual  maintenance fee through the life
          of the patents.

(12)      PHARMACIA BIOTECH, INC. COLLABORATION

          In December 1994, the Company and Pharmacia Biotech,  Inc. (Pharmacia)
          entered into a collaboration involving the design and development of a
          large-scale oligonucleotide synthesis machine. Following completion of
          the machine in December 1996, the collaboration expired, and Pharmacia
          retained the right to sell the machine to third parties, subject to an
          obligation  to pay the Company  royalties on such  third-party  sales.
          During  1996 and 1997,  the  Company  received  $62,321  and  $48,000,
          respectively, of royalty income related to such third-party sales. The
          Company recognized no royalty income related to this collaboration for
          1998.

(13)      PERKIN-ELMER CORPORATION SALES AND SUPPLY AGREEMENT

          In September 1996, the Company and the Applied Biosystems  Division of
          Perkin-Elmer  Corporation  (Perkin-Elmer) signed a four-year sales and
          supply  agreement under which  Perkin-Elmer  agreed to refer potential
          customers to HSP for the  manufacture of custom  oligonucleotides  and
          the  Company  agreed  that  amidites  for  the  manufacture  of  these
          oligonucleotides   would  be  purchased   from  Perkin-  Elmer  and  a
          percentage  of the  sales  price  will  be paid  to  Perkin-Elmer.  In
          addition,  Perkin-Elmer  licensed to the  Company its  oligonucleotide
          synthesis patents.

(14)      INVESTMENT IN METHYLGENE, INC.

          In  January  1996,  the  Company  and  three  Canadian   institutional
          investors formed a Quebec company,  MethylGene,  Inc.  (MethylGene) to
          develop and market certain  compounds and procedures to be agreed upon
          by the Company and MethylGene.



                                      F-17



<PAGE>

                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)

          The Company has granted to MethylGene exclusive worldwide licenses and
          sublicenses  in  respect  of  certain   technology   relating  to  the
          MethylGene  fields.  These  fields,  as  amended,  are  defined as (i)
          antisense compounds to inhibit DNA methyltransferase for the treatment
          of any disease; (ii) other methods of inhibiting DNA methyltransferase
          for the  treatment of any disease;  and (iii)  antisense  compounds to
          inhibit up to two  additional  molecular  targets for the treatment of
          cancers, to be agreed upon by the Company and MethylGene. In addition,
          the  Company  and  MethylGene  have  entered  into a supply  agreement
          pursuant to which MethylGene is obligated to purchase from the Company
          all required  formulated bulk  oligonucleotides  at specified transfer
          prices.

          The Company  acquired a 49% interest in MethylGene  for  approximately
          $734,000,  and the  Canadian  investors  acquired  a 51%  interest  in
          MethylGene for a total of approximately $5,500,000.  The institutional
          investors  have the right to  exchange  all (but not less than all) of
          their shares of stock in MethylGene for an aggregate of 100,000 shares
          of Hybridon  common stock  (subject to  adjustment  for stock  splits,
          stock dividends and the like).  This option is exercisable only during
          a 90-day period commencing on the earlier of the date five years after
          the closing of the institutional  investors'  investment in MethylGene
          or the  date  on  which  MethylGene  ceases  operations.  This  option
          terminates sooner if MethylGene raises certain  additional  amounts of
          equity or debt  financing  or if  MethylGene  enters  into a corporate
          collaboration that meets certain requirements. During 1998, MethylGene
          raised  additional  proceeds from outside investors that decreased the
          Company's   interest  to  30%.  The  Company  is  accounting  for  its
          investment  in  MethylGene  under the equity  method  and,  due to the
          existence of the investors  exchange rights, the Company has recorded,
          up to its  original  investment,  100% of  MethylGene's  losses in the
          accompanying consolidated statement of operations.

          In May  1998,  this  agreement  was  amended  to  grant  MethylGene  a
          non-exclusive   right  to  use  any  and  all  antisense   chemistries
          discovered  by the  Company  or any of  its  affiliates  for a  period
          commencing  on May 5,  1998  and  ending  on the  earlier  of (i)  the
          effective  date of  termination  by  MethylGene  of its  contract  for
          development services to be provided by the Company;  (ii) May 5, 1999,
          unless  MethylGene  exercises its option to continue  contracting  for
          development services provided by the Company; or (iii) May 5, 2000. As
          additional  consideration for this nonexclusive  right,  MethylGene is
          required to pay the Company certain milestone amounts, as defined, and
          transferred  300,000  shares  of  MethylGene's  Class B shares  to the
          Company. The Company has placed no value on these shares. During 1996,
          1997  and  1998,  the  Company   recognized   $49,565,   $101,894  and
          $1,685,932,  respectively,  of product and service  revenue related to
          this agreement.

(15)      STOCKHOLDERS' EQUITY (DEFICIT)

         (a)      Common Stock

                  The Company has 100,000,000 authorized shares of common stock,
                  $.001 par value,  of which  15,304,825  shares were issued and
                  outstanding at December 31, 1998.


                                      F-18



<PAGE>

                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)
         (b)      Initial Public Offering (IPO)

                  On  February  2,  1996,  the  Company  completed  its  IPO  of
                  1,150,000 shares of common stock at $50.00 per share. The sale
                  of common  stock  resulted  in net  proceeds to the Company of
                  $52,231,244 after deducting expenses related to the offering.

         (c)      1998 Unit Financing

                  On May 5, 1998,  the Company  completed a private  offering of
                  equity securities  raising total gross proceeds of $26,681,164
                  from the issuance of 9,597,476 shares of common stock, 114,285
                  shares of Series A convertible preferred stock and warrants to
                  purchase  3,329,486 shares of common stock at $2.40 per share.
                  The gross  proceeds  include the  conversion  of $5,934,558 of
                  accounts   payable,   capital  lease   obligations  and  other
                  obligations into common stock. The Company incurred $1,636,137
                  of cash  expenses  related to the private  offering and issued
                  597,699  shares  of  common  stock and  warrants  to  purchase
                  1,720,825  shares  of  common  stock at $2.40 per share to the
                  placement  agents.  The  compensation  received  by Pillar,  a
                  company affiliated with certain directors of the Company, with
                  respect to the  offshore  component  of the  private  offering
                  (Offshore  Offering)  consisted of (i) 9% of gross proceeds of
                  such  Offshore  Offerings  and (ii) a  nonaccountable  expense
                  allowance  equal  to 4% of  gross  proceeds  of such  Offshore
                  Offering.  Pillar received $1,636,137 and warrants to purchase
                  1,111,630 shares of common stock at $2.40 per share.

                  In addition,  Pillar is entitled to receive  300,000 shares of
                  common stock in  connection  with its efforts in assisting the
                  Company in  restructuring  its balance sheet.  The Company has
                  recorded $600,000 of general and administrative expense in the
                  accompanying consolidated statement of operations during 1998,
                  which represents the value of this common stock on May 5, 1998
                  with an offsetting  amount to accrued  expenses for the shares
                  to be issued. These shares will be issued in 1999.

         (d)      Units Issued to Primedica Corporation

                  In  connection  with the unit  financing  (see Note 15(c)) the
                  Company  issued  250,000  shares  of common  stock and  62,500
                  warrants to purchase  common  stock to  Primedica  Corporation
                  (Primedica) for future  services to be provided.  The services
                  shall  commence  upon  the  Company's  request  after  (i) the
                  Company's  securities  are listed on a  nationally  recognized
                  exchange,  and (ii) the average closing price of the Company's
                  common  stock is at least  $2.00 per share for the  twenty-day
                  trading period  preceding the contract  commencement  date. In
                  the event that the  Company  does not use these  services as a
                  result  of  the  failure  to  meet  the  contract  conditions,
                  Primedica  shall  forfeit  to the  Company  all or part of the
                  common stock and warrants held by  Primedica.  The Company has
                  recorded  these shares as issued and  outstanding  at December
                  31, 1998 at par value.  The  Company  will record the value of
                  these services as the services are rendered.


                                      F-19


         (e)      Stock Split

                  On  December  10,  1997,  the Board of  Directors  declared  a
                  one-for-five   reverse  split  of  its  common  stock.   Share
                  quantities   and   related   per  share   amounts   have  been
                  retroactively restated to reflect the reverse stock split.

         (f)      Warrants

                  The  Company  has  the  following  warrants   outstanding  and
                  exercisable  for the  purchase of common stock at December 31,
                  1998:


<PAGE>

                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)
<TABLE>
<CAPTION>

                                       Outstanding         Exercise Price        Exercisable        Exercise Price
Expiration Date                         Warrants             per Share            Warrants             per Share

<S>                                      <C>                  <C>                  <C>                 <C>
February 4, 1999-October 25, 2000         551,201             $  50.00              551,201           $  50.00
February 28, 2000                          20,000                37.50               20,000              37.50
December 31, 2001                          13,000                34.49               13,000              34.49
May 4, 2003                             8,641,503            2.40-4.25            4,378,044               2.40
                                        ---------            ---------            ---------           --------
                                        9,225,704                                 4,962,245
                                        =========                                 =========

Weighted average exercise price per                              $5.48                                   $7.91
share                                                            =====                                   =====
</TABLE>

                  Five-year  warrants to purchase 368,620 shares of common stock
                  at  $50.00  per  share  were  issued  in  1994  and  1995 as a
                  component  of  the   compensation   for  services  of  several
                  placement agents of the Company's convertible preferred stock.
                  Of these  warrants,  304,335  were issued to a company that is
                  controlled  by two  directors of the Company (see Note 16(b)).
                  The  remaining  64,285  warrants  were issued to various other
                  companies that acted as placement  agents.  See Note 15(c) for
                  information relating to warrants issued to placement agents in
                  connection with the 1998 Unit Financing.

                  As consideration of the agreements made by Forum consenting to
                  the  Company's  1998 private  placements  and waiving  certain
                  obligations  of the Company to Forum,  the  Company  agreed to
                  amend the warrant to purchase 71,301 shares of common stock at
                  an  exercise  price of $35.06  per  share,  issued to Forum in
                  connection  with 9% notes so that the  exercise  price will be
                  equal to $4.25 per  share,  and the number of shares of common
                  stock  purchasable  upon exercise thereof will be increased to
                  588,235,  in  each  case  subject  to  adjustment;   provided,
                  however,  that such  warrant  will also be  amended to provide
                  that such warrant may not be  exercised  until May 5, 1999 and
                  the transactions  contemplated by such private  placements and
                  by the  exchange  offer  will not  trigger  any  anti-dilution
                  adjustments  to the  exercise  price  thereof or the number of
                  shares of common stock subject thereto.

         (g)      Stock Options

                  In 1990 and  1995,  the  Company  established  the 1990  Stock
                  Option Plan (the 1990 Option  Plan) and the 1995 Stock  Option
                  Plan (the 1995 Option Plan),  respectively,  which provide for
                  the grant of incentive  stock options and  nonqualified  stock
                  options.  Options  granted
                                      F-20



<PAGE>

                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)

                  under  these  plans vest over  various  periods  and expire no
                  later than 10 years from the date of grant. However, under the
                  1990  Option  Plan,  in the event of a change in  control  (as
                  defined in the 1990 Plan),  the exercise  dates of all options
                  then  outstanding   shall  be  accelerated  in  full  and  any
                  restrictions on exercising outstanding options issued pursuant
                  to the 1990 Option Plan shall terminate.  In October 1995, the
                  Company  terminated  the issuance of additional  options under
                  the 1990 Option  Plan.  As of December  31,  1998,  options to
                  purchase a total of 525,638  shares of common  stock  remained
                  outstanding under the 1990 Option Plan.

                  A total of 700,000  shares of common  stock may be issued upon
                  the  exercise of options  granted  under the 1995 Option Plan.
                  The maximum number of shares with respect to which options may
                  be granted to any  employee  under the 1995  Option Plan shall
                  not exceed  500,000 shares of common stock during any calendar
                  year. The Compensation Committee of the



                  Board of Directors  has the  authority to select the employees
                  to whom  options are granted and  determine  the terms of each
                  option,  including  (i) the  number of shares of common  stock
                  subject  to  the   option;   (ii)  when  the  option   becomes
                  exercisable;  (iii) the option exercise  price,  which, in the
                  case of incentive  stock options,  must be at least 100% (110%
                  in  the  case  of  incentive   stock  options   granted  to  a
                  stockholder  owning in excess of 10% of the  Company's  common
                  stock) of the fair market  value of the common stock as of the
                  date of grant; and (iv) the duration of the option (which,  in
                  the case of incentive stock options, may not exceed 10 years).
                  As of  December  31,  1998,  options  to  purchase  a total of
                  550,534 shares of common stock remained  outstanding under the
                  1995 Option Plan.

                  In October 1995,  the Company  adopted the 1995 Director Stock
                  Option Plan (the Director  Plan).  A total of 50,000 shares of
                  common  stock  may be  issued  upon the  exercise  of  options
                  granted  under  the  Director  Plan.  Under  the  terms of the
                  Director  Plan,  options to  purchase  1,000  shares of common
                  stock were granted to eligible  directors  upon the closing of
                  the Company's initial public offering at the fair market value
                  of the common  stock on the date of the  closing.  Thereafter,
                  options  to  purchase  1,000  shares of common  stock  will be
                  granted  to  each  eligible  director  on May 1 of  each  year
                  commencing  in  1997.  All  options  will  vest  on the  first
                  anniversary  of the date of grant  or,  in the case of  annual
                  options,  on April 30 of each year  with  respect  to  options
                  granted in the previous year. As of December 31, 1998, options
                  to purchase a total of 21,000 shares of common stock  remained
                  outstanding under the Director Plan.

                  In May 1997,  the Company  adopted the 1997 Stock  Option Plan
                  (the 1997 Option  Plan) and has  reserved  and may issue up to
                  4,500,000  shares for the grant of incentive and  nonqualified
                  stock  options.  The maximum  number of shares with respect to
                  which  options may be granted to any  employee  under the 1997
                  Option Plan shall not exceed  500,000  shares of common  stock
                  during any calendar  year. The  Compensation  Committee of the
                  Board of Directors  has the  authority to select the employees
                  to whom  options are granted and  determine  the terms of each
                  option,  including  (i) the  number of shares of common  stock
                  subject  to  the   option;   (ii)  when  the  option   becomes
                  exercisable;  (iii) the option exercise  price,  which, in the
                  case of incentive  stock options,  must be at least 100% (110%
                  in the case of  incentive  stock) of the fair market  value of
                  the  common  stock  as of the  date of  grant;  and  (iv)  the
                  duration of the option (which,  in the case of incentive stock
                  options,
                                      F-21



<PAGE>

                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)

                  may not exceed ten years). As of December 31, 1998, options to
                  purchase a total of 2,363,560  shares of common stock remained
                  outstanding under the 1997 Option Plan.



                  Stock option  activity for the three years ended  December 31,
1998 is summarized as follows:
<TABLE>
<CAPTION>

                                                                                             Weighted
                                               Number          Exercise Price               Average Price
                                             of Shares           per Share                   per Share

<S>                                             <C>           <C>         <C>                  <C>
Outstanding, December 31, 1995                 738,208       $ .01    -   $50.00              $29.15
     Granted                                   476,020       25.00    -    65.60               49.55
     Exercised                                 (57,740)        .01    -    37.50               18.85
     Terminated                                (20,100)      25.00    -    57.85               40.20
                                               -------

Outstanding, December 31, 1996               1,136,388        1.25    -    65.60               38.05
     Granted                                   315,675       27.50    -    32.50               30.75
     Exercised                                 (25,005)       1.25    -    40.00               12.60
     Terminated                               (236,561)       2.50    -    65.60               40.35
                                              --------

Outstanding, December 31, 1997               1,190,497        1.25    -    65.60               36.18
     Granted                                 2,513,000        2.00    -     3.13                2.00
     Terminated                               (242,765)       2.50    -    57.85               37.79
                                              --------

Outstanding, December 31, 1998               3,460,732       $1.25    -  $ 65.60              $11.25
                                             =========       ===================              ======

Exercisable, December 31, 1996                 622,930       $1.25    -  $ 65.60              $32.55
                                             =========       ===================              ======

Exercisable, December 31, 1997                 740,780       $1.25    -  $ 65.50              $34.40
                                             =========       ===================              ======

Exercisable, December 31, 1998               1,650,021       $1.25    -  $ 65.60              $17.13
                                             =========       ===================              ======
</TABLE>

<TABLE>
<CAPTION>

                                              Options Outstanding                        Options Exercisable

                                                  Weighted       Weighted                             Weighted
                                                  Average         Average                             Average
                                                 Remaining       Exercise                             Exercise
      Range of Exercise          Number         Contractual      Price per          Number           Price per
            Prices            Outstanding          Life            Share          Outstanding          Share
<S>               <C>           <C>                <C>          <C>                 <C>             <C>

$                 1.25           10,000            3.10        $    1.25             10,000       $      1.25
    2.00    -     2.37        2,505,000            9.56             2.00            901,562              2.00
    2.44    -     3.13           18,800            6.03             2.61             10,800              2.50
    4.25    -     5.00            1,200            3.75             5.00              1,200              3.75
   17.50    -     25.00         197,330            3.54            23.21            191,331             23.15
   27.50    -     31.66         168,974            7.45            30.50             76,017             30.28
   35.00    -     36.25          30,000            6.73            35.71             30,000             35.71
   37.50    -     37.50         316,048            4.72            37.50            282,583             37.50
   38.13    -     43.75          47,900            7.81            40.64             24,648             40.73
                  50.00          17,700            6.35            50.00             11,700             50.00
   57.85    -     65.60         147,780            6.08            58.22            110,180             58.34
                              ---------                         --------

                              3,460,732                         $  11.25          1,650,021       $     17.13
                              =========                         ========          =========       ===========
</TABLE>


                                      F-22



<PAGE>

                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)

                  In October 1995, the FASB issued SFAS No. 123,  Accounting for
                  Stock-Based   Compensation.   SFAS  No.   123   requires   the
                  measurement  of the fair value of stock  options  or  warrants
                  granted  to  employees  to be  included  in the  statement  of
                  operations or disclosed in the notes to financial  statements.
                  The Company has  determined  that it will  continue to account
                  for stock-based  compensation  for employees under  Accounting
                  Principles Board Opinion No. 25 and elect the  disclosure-only
                  alternative  under SFAS No. 123. In 1996,  1997 and 1998,  the
                  Company   recorded   $1,967,116,    $205,978   and   $109,734,
                  respectively,  of deferred  compensation  related to grants to
                  nonemployees, net of terminations.  Deferred compensation will
                  be  amortized  over the  vesting  period of the  options.  The
                  Company  has  recorded   compensation   expense  of  $763,190,
                  $316,067  and $246,444 in 1996,  1997 and 1998,  respectively,
                  related to these grants to nonemployees.

                  The Company has computed the pro forma disclosures  require by
                  SFAS No. 123 for all stock  options  granted  after January 1,
                  1995  using  the  Black-Scholes   option  pricing  model.  The
                  assumptions  used for the three years ended  December 31, 1998
                  are as follows:


                                          1996          1997         1998

          Risk free interest rate         6.14%         6.22%         5.15%
          Expected dividend yield            -             -             -
          Expected lives                 6 years       6 years       6 years
          Expected volatility              60%           60%            60%


                  The  Black-Scholes  option pricing model was developed for use
                  in estimating  the fair value of traded  options which have no
                  vesting restrictions and are fully transferable.  In addition,
                  option pricing  models require the input of highly  subjective
                  assumptions including expected stock price volatility. Because
                  the  Company's  employee  stock  options have  characteristics
                  significantly  different  from  those of traded  options,  and
                  because  changes  in  the  subjective  input  assumptions  can
                  materially  affect the fair value  estimate,  in  management's
                  opinion,  the  existing  models do not  necessarily  provide a
                  reliable  single  measure  of the fair  value of its  employee
                  stock options.

                  The effect of applying  SFAS No. 123 for the three years ended
                  December 31, 1998 would be as follows:

                                    1996             1997           1998
Net loss applicable to common
stockholders-
  As reported                    $(46,852,600)    $(69,461,326)   $(19,792,736)
                                 ============
  Pro forma                      $(52,890,455)    $(73,402,170)   $(23,131,304)
                                 ============     ============    ============
Basic and Diluted net loss per
common shares-
  As reported                      $(10.24)         $(13.76)          $(1.67)
                                   =======          =======           ======
  Pro forma                        $(11.56)         $(14.54)          $(1.95)
                                   =======          =======           ======



                                      F-23



<PAGE>

                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)

        (h)       Employee Stock Purchase Plan

                  In October 1995,  the Company  adopted the 1995 Employee Stock
                  Purchase Plan (the Purchase  Plan),  under which up to 100,000
                  shares  of  common  stock  may  be  issued  to   participating
                  employees of the Company, as defined, or its subsidiaries.

                  On the first day of a designated payroll deduction period (the
                  Offering  Period),  the  Company  will grant to each  eligible
                  employee who has elected to  participate  in the Purchase Plan
                  an option to purchase  shares of common stock as follows:  the
                  employee may authorize an amount (a whole  percentage  from 1%
                  to 10% of such  employee's  regular pay) to be deducted by the
                  Company from such pay during the Offering Period.  On the last
                  day of the  Offering  Period,  the  employee is deemed to have
                  exercised the option,  at the option  exercise  price,  to the
                  extent of accumulated payroll  deductions.  Under the terms of
                  the Purchase  Plan, the option price is an amount equal to 85%
                  of the fair  market  value  per share of the  common  stock on
                  either the first day or the last day of the  Offering  Period,
                  whichever  is lower.  In no event may an employee  purchase in
                  any one Offering  Period a number of shares which is more than
                  15% of the  employee's  annualized  base pay divided by 85% of
                  the  market   value  of  a  share  of  common   stock  on  the
                  commencement  date of the Offering  Period.  The  Compensation
                  Committee may, in its discretion, choose an Offering Period of
                  12  months  or less for  each of the  Offerings  and  choose a
                  different  Offering  Period for each Offering.  No shares have
                  been issued under the Plan.

        (i)       Preferred Stock

                  The  restated  Certificate  of  Incorporation  of the  Company
                  permits its Board of Directors to issue up to 5,000,000 shares
                  of preferred  stock,  par value $.01 per share (the  Preferred
                  Stock),  in one or more  series,  to  designate  the number of
                  shares  constituting such series,  and fix by resolution,  the
                  powers,  privileges,  preferences  and  relative,  optional or
                  special rights thereof,  including liquidation preferences and
                  dividends,  and conversion and redemption  rights of each such
                  series.  During 1998, the Company designated  1,500,000 shares
                  as Series A convertible preferred stock.

        (j)       Series A Convertible Preferred Stock

                  The  rights  and  preferences  of  the  Series  A  convertible
                  preferred stock are as follows:

                  Dividends

                  The holders of the Series A convertible preferred stock, as of
                  March 15 or September  15, are  entitled to receive  dividends
                  payable at the rate of 6.5% per annum,  payable semi- annually
                  in  arrears.  Such  dividends  shall  accrue  from the date of
                  issuance  of such  share  and shall be paid  semi-annually  on
                  April 1 and October 1 of each year.  Such  dividends  shall be
                  paid,  at the  election  of the  Company,  either  in  cash or
                  additional  duly  authorized,  fully  paid and non  assessable
                  shares of Series A convertible preferred stock. In calculating
                  the number of shares of Series A convertible  preferred  stock
                  to be  paid  with  respect  to each  dividend,  the  Series  A
                  convertible preferred stock
                                      F-24



<PAGE>

                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)

                  shall be valued at $100.00 per share. During 1998, the Company
                  recorded a total  accretion of $2,689,048  for the dividend on
                  Series A preferred  stock and issued 16,470 shares of Series A
                  convertible preferred stock as a dividend.

                  Liquidation

                  In the event of a  liquidation,  dissolution  or winding up of
                  the Company,  whether voluntary or involuntary,  after payment
                  or provision for payment of debts and other liabilities of the
                  Company,  the  holder of the  Series A  convertible  preferred
                  stock then outstanding shall be entitled to be paid out of the
                  assets  of  the  Company  available  for  distribution  to its
                  stockholders,  an amount  equal to $100.00  per share plus all
                  accrued but unpaid dividends.  If the assets to be distributed
                  to the  holders of the Series A  convertible  preferred  stock
                  shall  be  insufficient  to  permit  the  payment  of the full
                  preferential  amounts, then the assets of the Company shall be
                  distributed ratably to the holders of the Series A convertible
                  preferred stock on the basis of the number of shares of Series
                  A  convertible  preferred  stock held.  All shares of Series A
                  convertible  preferred stock shall rank as to payment upon the
                  occurrence  of any  liquidation  event  senior  to the  common
                  stock.

                  Conversion

                  Commencing  after  May 6,  1999,  but not prior  thereto,  the
                  shares  of  Series  A  convertible  preferred  stock  shall be
                  convertible,  in whole or in part, at the option of the holder
                  into fully paid and  nonassessable  shares of common  stock at
                  $4.25 per share, subject to adjustment as defined.

                  Mandatory Conversion

                  At any time after May 6, 1998, the Company at its option,  may
                  cause the Series A convertible preferred stock to be converted
                  in whole or in part, on a pro rata basis,  into fully paid and
                  nonassessable  shares of common stock using a conversion price
                  equal to $4.00 if the  closing bid price,  as defined,  of the
                  common  stock  shall  have  equaled  or  exceeded  250% of the
                  conversion price, $4.25, subject to adjustment as defined, for
                  at least 20 trading  days in any 30  consecutive  trading  day
                  period  ending  three  days  prior  to the date of  notice  of
                  conversion (such event, the Market Trigger).

                  At any time after April 1, 2000,  the Company,  at its option,
                  may redeem the Series A convertible  preferred  stock for cash
                  equal to  $100.00  per  share  plus  all  accrued  and  unpaid
                  dividends at such time, if the Market  Trigger has occurred in
                  the  period  ending  three days prior to the date of notice of
                  redemption.

(16)        COMMITMENTS AND CONTINGENCIES

            (a)         Facilities

                        The   Company    leases   its   facility   in   Milford,
                        Massachusetts,  under a lease which has a 10- year term,
                        which commenced on July 1, 1994, with certain  extension
                        options.

                                      F-25



<PAGE>

                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)


                  On February 4, 1994,  the Company  entered into the  Cambridge
                  Lease  with a  partnership  that is  affiliated  with  certain
                  directors of the Company.  As compensation  for arranging this
                  lease,  the Company issued Pillar Limited  five-year  warrants
                  for the  purchase of 100,000  shares of the  Company's  common
                  stock at an exercise price of $50.00 per share. These warrants
                  expired  subsequent to December 31, 1998. The Company  vacated
                  the Cambridge, Massachusetts,  facility in June 1998 and moved
                  its corporate  facilities to Milford,  Massachusetts (see Note
                  3).

                  Future  approximate  minimum rent  payments as of December 31,
                  1998,  under  existing lease  agreements  through 2007, net of
                  sublease agreements are as follows:

                     December 31,                    Amount

                     1999                        $   614,000
                     2000                            784,000
                     2001                          1,213,000
                     2002                          1,209,000
                     2003                          1,213,000
                     Thereafter                    2,338,000
                                                 -----------

                                                 $ 7,371,000
                                                 ===========


                  During  1996,  1997 and 1998,  facility  rent  expense  net of
                  sublease revenue was approximately $2,352,000,  $4,613,000 and
                  $3,871,000, respectively.

            (b)   Related-Party  Agreements with Affiliates of Stockholders  and
                  Directors

                  The Company  has entered  into  consulting  agreements,  stock
                  placement  agreements  and an advisory  agreement with several
                  companies  that  are  controlled  by  two   shareholders   and
                  directors  of  the  Company   including  Forum,   S.A.  Pillar
                  Investment N.V.  (Pillar  Investment),  Pillar S.A.  (formerly
                  Commerce Consult S.A.) and Pillar Investment Limited (formerly
                  Ash Properties  Limited) (Pillar  Limited).  During 1996, 1997
                  and 1998,  the Company had expensed  $1,106,000,  $998,000 and
                  $1,300,000,   respectively,   under  consulting  and  advisory
                  agreements with related parties.

            (c)   Other Research and Development Agreements

                  The  Company  has  entered   into   consulting   and  research
                  agreements  with  the   universities,   research  and  testing
                  organizations  and  individuals,  under which  consulting  and
                  research support is provided to the Company.  These agreements
                  are for varying terms and provide for certain  minimum  annual
                  or per diem fees plus reimbursable  expenses to be paid during
                  the contract periods.  Future minimum fees payable under these
                  contracts  as  of  December  31,  1998  are  approximately  as
                  follows:


                                      F-26



<PAGE>

                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)

                      December 31,               Amount

                      1999                   $    582,000
                      2000                        392,000
                      2001                        279,000
                                             ------------

                                             $  1,253,000
                                             ============


                  Total  fees  and   expenses   under   these   contracts   were
                  approximately  $7,171,000,  $9,372,000 and  $2,011,000  during
                  1996, 1997 and 1998, respectively.

            (d)   Employment Agreements

                  The Company has entered into  employment  agreements  with its
                  executive officers which provide for, among other things, each
                  officer's  annual salary,  cash bonus,  fringe  benefits,  and
                  vacation and severance arrangements. Under the agreements, the
                  officers are generally  entitled to receive severance payments
                  of two to three year's base salary.

            (e)   Contingencies

                  From time to time, the Company may be exposed to various types
                  of  litigation.  The  Company  is not  engaged  in  any  legal
                  proceedings   that  are  expected,   individually  or  in  the
                  aggregate,  to have a material adverse effect on the Company's
                  financial condition or results of operations.

(17)        INCOME TAXES

            The Company  applies SFAS No. 109,  Accounting for Income Taxes.  At
            December 31, 1998, the Company had net operating loss and tax credit
            carryforwards  for  federal  income tax  purposes  of  approximately
            $219,993,000  and  $3,936,000,  respectively,  available  to  reduce
            federal taxable income and federal income taxes,  respectively.  The
            Tax Reform Act of 1986 (the Act),  enacted in October  1986,  limits
            the  amount of net  operating  loss and  credit  carryforwards  that
            companies  may  utilize  in any one year in the event of  cumulative
            changes in ownership over a three-year  period in excess of 50%. The
            Company has completed several financings since the effective date of
            the Act,  which, as of December 31, 1998, have resulted in ownership
            changes in excess of 50%,  as  defined  under the Act and which will
            limit the  Company's  ability  to  utilize  its net  operating  loss
            carryforwards.   Ownership  changes  in  future  periods  may  place
            additional  limits on the Company's ability to utilize net operating
            loss and tax credit carryforwards.

            The  federal  net  operating  loss   carryforwards  and  tax  credit
            carryforwards expire approximately as follows:


                                      F-27



<PAGE>

                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)

                                             Net
                                        Operating Loss         Tax Credit
             Expiration Date            Carryforwards        Carryforwards

              December 31,
                2005                  $      666,000         $   15,000
                2006                       3,040,000             88,000
                2007                       7,897,000            278,000
                2008                      18,300,000            627,000
                2009                      25,670,000            689,000
                2010                      36,134,000            496,000
                2011                      44,947,000            493,000
                2012                      60,087,000            750,000
                2018                      23,252,000            500,000
                                      --------------        -----------
                                      $  219,993,000        $ 3,936,000
                                      ==============        ===========

            At December 31, 1997 and 1998,  the  components  of the deferred tax
            assets are approximately as follows:

                                                   1997               1998

              Operating loss carryforwards     $  78,696,000     $  87,997,000
              Temporary differences                5,137,000         2,677,000
              Tax credit carryforwards             3,436,000         3,936,000
                                               -------------     -------------
                                                  87,269,000        94,610,000

              Valuation allowance                (87,269,000)      (94,610,000)
                                               -------------     -------------
                                               $           -     $           -
                                               =============     =============

            A valuation  allowance has been provided,  as it is more likely than
            not the Company  will not realize the  deferred  tax asset.  The net
            change in the total valuation  allowance during 1998 was an increase
            of approximately $7,341,000.

(18)        EMPLOYEE BENEFIT PLAN

            On October 10, 1991,  the Company  adopted an employee  benefit plan
            under Section  401(k) of the Internal  Revenue Code. The plan allows
            employees  to make  contributions  up to a specified  percentage  of
            their  compensation.  Under the plan,  the Company  may,  but is not
            obligated to, match a portion of the employees'  contributions up to
            a defined maximum. The Company is currently matching 50% of employee
            contributions  to the plan, up to 6% of the  employee's  annual base
            salary, and charged to operations  approximately $224,000,  $253,000
            and $253,000 during 1996, 1997 and 1998, respectively.


                                      F-28



<PAGE>

                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)

(19)        SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION

            Supplemental disclosure of cash flow information for the three years
            in the period ended December 31, 1998 are as follows:
<TABLE>
<CAPTION>

                                                                                     1996               1997              1998

<S>                                                                              <C>                <C>               <C>
              Cash paid during the period for interest                           $   124,052        $  3,264,596      $  1,666,127
                                                                                 ============       ============      ============

              Purchase of property and equipment under capital leases            $ 1,722,333        $  2,374,502      $          -
                                                                                 ============       ============      ============

              Conversion of preferred stock into common stock                    $   159,822        $          -      $          -
                                                                                 ============       ============      ============

              Deferred compensation related to grants of stock options to
                nonemployees, net of terminations                                $ 1,967,116        $    205,978      $    109,734
                                                                                 ============       ============      ============

              Issuance of Series A convertible preferred stock and attached
                warrants in exchange for conversion of 9% convertible
                subordinated notes payable and accrued interest                  $          -       $          -      $ 51,055,850
                                                                                 ============       ============      ============

              Accretion of Series A convertible preferred stock dividends        $          -       $          -      $  2,689,048
                                                                                 ============       ============      ============

              Issuance of common stock and attached warrants in exchange
                for conversion of convertible promissory notes payable           $          -       $          -      $  4,800,000
                                                                                 ============       ============      ============

              Issuance of common stock and attached warrants in exchange
                for conversion of accounts payable and other obligations         $          -       $          -      $  5,934,558
                                                                                 ============       ============      ============
</TABLE>

(20)        RESTATEMENT

            In March 1999, the Company  restated its June 30, 1998 and September
            30, 1998 financial statements to reflect the accretion on the Series
            A convertible  preferred  stock,  and record $600,000 of general and
            administrative  expense for the 300,000  shares of common stock that
            Pillar is  entitled  to receive in  connection  with its  efforts in
            assisting the Company in restructuring its balance sheet.

(21)        ORIGENIX TECHNOLOGIES, INC.

            In January  1999,  the Company and certain  institutional  investors
            formed a Montreal company, OriGenix Technologies Inc. (OriGenix), to
            develop and market drugs for the treatment of infectious diseases.

            The Company  received a 49% interest in OriGenix in consideration of
            certain research and development  efforts  previously  undertaken by
            the Company which were made  available to OriGenix.  The Company has
            also licensed  certain  antisense  compounds and other technology to
            OriGenix.  If certain  conditions  are  satisfied by  OriGenix,  the
            institutional   investors   are  committed  to  make  an  additional
            investment,  at  which  time the  Company's  ownership  interest  in
            OriGenix will be reduced 40%. The institutional investors acquired a
            51% interest in OriGenix for a total of approximately  $4.0 million.
            The Company will account for its  investment  in OriGenix  under the
            equity method.


                                      F-29



<PAGE>

                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)


(22)        INTERIM PERIOD AND SUBSEQUENT EVENTS (Unaudited)

            (a)   Unaudited Interim Financial Statements

                  The  accompanying  consolidated  balance sheet as of March 31,
                  1999,   and  the   consolidated   statements  of   operations,
                  stockholders'  equity  (deficit)  and cash flows for the three
                  months  ended March 31, 1998 and 1999 are  unaudited,  but, in
                  the  opinion  of  management,  have been  prepared  on a basis
                  substantially consistent with audited financial statements and
                  include all  adjustments,  consisting of only normal recurring
                  adjustments,  necessary for a fair presentation of the results
                  of these  interim  periods.  The results for the period  ended
                  March 31, 1999  presented  are not  necessarily  indicative of
                  results to be expected for the full fiscal year.

                  At March 31, 1999,  the Company had cash and cash  equivalents
                  of approximately $2.5 million and a working capital deficit of
                  approximately  $7.8 million.  The Company is currently seeking
                  debt or equity  financing in an amount  sufficient  to support
                  its  operations  through  the end of 1999,  and in  connection
                  therewith,  is in negotiations  with several parties to obtain
                  such  financing.  The Company's  existing  cash  resources and
                  proceeds  of  accounts   receivable  from  HSP  customers  are
                  expected to be  sufficient  to fund the  Company's  operations
                  into  July  1999.  The  Company's   management   expects  such
                  receivables  to be  collected  no later than July 1999,  given
                  such customers'  payment  histories,  although there can be no
                  assurance  thereof.   If  the  Company  is  unable  to  obtain
                  additional  funding by the end of July 1999, it will be forced
                  to terminate its  operations  or seek relief under  applicable
                  bankruptcy law.

            (b)   Net Loss per Common Share

                  The Company  applies  SFAS No.  128,  Earnings  per Share,  in
                  calculating  earnings  per share.  Basic net loss per share is
                  computed   by   dividing   net  loss   applicable   to  common
                  stockholders  by the weighted  average number of common shares
                  outstanding during the period.  Diluted net loss per share for
                  the periods  presented is the same as basic net loss per share
                  as the  inclusion of the  potential  common stock  equivalents
                  would be antidilutive.  Antidilutive  securities which consist
                  of stock options, warrants and convertible preferred stock (on
                  an  as-converted  basis) that are not  included in diluted net
                  loss per common share were  2,220,880 and  29,301,825  for the
                  three   month   periods   ended   March  31,  1998  and  1999,
                  respectively.

            (c)   Comprehensive Loss

                  The Company follows the provisions of SFAS No. 130,  Reporting
                  Comprehensive  Income.  Comprehensive  loss is  defined as the
                  change in equity of a business enterprise during a period from
                  transactions and other events and circumstances  from nonowner
                  sources.  The Company's  comprehensive loss is the same as the
                  reported net loss for all periods presented.


                                      F-30



<PAGE>

                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)

            (d)   Cash Equivalents

                  The  Company  considers  all highly  liquid  investments  with
                  maturities  of three  months  or less to be cash  equivalents.
                  Cash and cash  equivalents  at March 31, 1999 consisted of the
                  following (at amortized cost, which  approximates  fair market
                  value):

                                                         March 31,
                                                           1999
              Cash and cash equivalents-
                    Cash and money market funds         $2,267,758
                    Corporate bond                         193,426
                                                        ----------
                                                        $2,461,184
                                                        ==========

            (e)   Note Payable to Lenders

                  In  December  1996,  the  Company  entered  into  a  five-year
                  $7,500,000  note  payable to a bank.  In  November  1998,  the
                  outstanding balance of approximately  $2,895,000 was purchased
                  from the bank by Forum  Capital  Markets,  LLC  ("Forum")  and
                  certain  investors  associated with Pecks Management  Partners
                  Ltd.  ("Pecks")  (collectively,   the  "Lenders"),  which  are
                  affiliates of two members of the Company's Board of Directors.
                  In  connection   with  the  purchase,   the  Lenders  lent  an
                  additional  $3,200,000  so  as  to  increase  the  outstanding
                  principal  amount of the note to $6,000,000.  The terms of the
                  note  payable  were  amended as follows:  (i) the maturity was
                  extended to November  30,  2003;  (ii) the  interest  rate was
                  decreased to 8%; (iii) interest is payable monthly in arrears,
                  with the principal  due in full at maturity of the loan;  (iv)
                  the note payable is convertible,  at the Lenders'  option,  in
                  whole or in part,  into shares of common stock at a conversion
                  price of $2.40  per  share;  (v) the note  includes  a minimum
                  liquidity,  as defined,  covenant of $2,000,000;  and (vi) the
                  note payable may not be prepaid,  in whole or in part,  at any
                  time prior to  December  1, 2000.  The  Company  has  received
                  waivers of  noncompliance  with the minimum tangible net worth
                  covenant  through the quarter ended March 31, 1999 and for the
                  minimum  liquidity  covenant for May 30, 1999. The Company has
                  classified the outstanding  balance of $6,000,000 at March 31,
                  1999 as a current  liability in the accompanying  consolidated
                  balance  sheet as it does not  expect to remain in  compliance
                  with the financial covenants.  In connection with the purchase
                  of the note payable,  Forum received  $400,000 as a fee, which
                  Forum has  reinvested by purchasing  160,000  shares of common
                  stock and warrants to purchase  40,000  shares of common stock
                  at $3.00 per share. The Company has recorded the $400,000 as a
                  deferred  financing cost,  which will be amortized to interest
                  expense  over the term of the note and an accrued  expense for
                  the issuance of common stock and warrants. In addition,  Forum
                  received  warrants to purchase  133,333 shares of common stock
                  of the Company at $3.00 per share.  The Company  recorded  the
                  value  of  the   warrants   to  be   $85,433,   by  using  the
                  Black-Scholes  option pricing model.  The Company has recorded
                  this  $85,433  as a  deferred  financing  cost,  which will be
                  amortized to interest expense over the term of the note.


                                      F-31



<PAGE>

                         HYBRIDON, INC. AND SUBSIDIARIES
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                (Including Data Applicable To Unaudited Periods)
                                   (continued)

            (h)   Accrued Expenses

                  Accrued   expenses  as  of  March  31,  1999  consist  of  the
                  following:

                                                              March 31, 1999
                  ----------------------------------------------------------
                  Restructuring                               $    486,000
                  Interest -                                        58,770
                  Payroll and related costs                      1,147,601
                  Outside research and clinical costs              460,419
                  Professional fees                                149,957
                  Contingent stock                               1,000,000
                  Other                                            450,382
                                                              ------------
                                                              $  3,753,129
                                                              ============
            (i)   Commitments

                  The Company is currently  undergoing a sales and use tax audit
                  by the Massachusetts  Department of Revenue. The amount of the
                  final assessment, while currently unknown, may be material.

            (j)   Supplemental Disclosure of Cash Flow Information

                  Supplemental disclosure of cash flow information for the three
                  month periods ended March 31, 1998 and 1999 are as follows:
<TABLE>
<CAPTION>

                                                                  Three Months Ended
                                                                       March 31,
                                                           -----------------------------
                                                              1998               1999
<S>                                                        <C>               <C>
Cash paid during the period for interest                   $  358,680        $   186,695
                                                           ==========        ===========

Accretion of Series A convertible preferred stock          $        -        $ 1,042,052
dividends                                                  ==========        ===========
</TABLE>



                                      F-32



<PAGE>

No  person  has  been  authorized  to  give  any  information  or  to  make  any
representations other than those contained in this Prospectus,  and, if given or
made, such information or representations must not be relied upon as having been
authorized.  This  Prospectus  does  not  constitute  an  offer  to  sell or the
solicitation  of an offer to buy any  securities  other than the  securities  to
which it relates or an offer to sell or the solicitation of an offer to buy such
securities in any circumstances in which such offer or solicitation is unlawful.
Neither the delivery of this Prospectus nor any sale made hereunder shall, under
any  circumstances,  create any implication that there has been no change in the
affairs of  Hybridon  since the date  hereof or that the  information  contained
herein is correct as of any time subsequent to its date. ---------------

                                TABLE OF CONTENTS
<TABLE>
<CAPTION>

                                                                                                Page
                                                                                                ----
<S>                                                                                             <C>
Prospectus Summary                                                                                3
Risk Factors                                                                                      5
Forward-Looking Statements                                                                       10
Business                                                                                         11
Properties                                                                                       23
Legal Proceedings                                                                                23
Market for Registrant's Common Equity and Related Stockholder Matters                            23
Dividend Policy                                                                                  24
Use of Proceeds                                                                                  25
Capitalization                                                                                   25
Selected Financial Data                                                                          26
Management's Discussion and Analysis of Financial Condition and Results of Operations            28
Directors and Executive Officers of Hybridon                                                     34
Executive Compensation                                                                           37
Security Ownership of Certain Beneficial Owners and Management                                   41
Certain Relationships and Related Transactions                                                   47
Selling Stockholders                                                                             51
Description of Capital Stock                                                                     57
Delaware Law and Certain Provisions of Hybridon's Restated Certificate of Incorporation,
  By-Laws and Indebtedness                                                                       60
Plan of Distribution                                                                             61
Legal Matters                                                                                    61
Experts                                                                                          62
Index to Consolidated Financial Statements                                                       F1
</TABLE>

                                 HYBRIDON, INC.


                                 662,099 SHARES

                      SERIES A CONVERTIBLE PREFERRED STOCK
                           ($.01 par value per share)


                                35,048,809 SHARES

                                  COMMON STOCK
                           ($.001 par value per share)




<PAGE>

                                     PART II

                     INFORMATION NOT REQUIRED IN PROSPECTUS


Item 13.  Other Expenses of Issuance and Distribution.

            Estimated   expenses   (other  than   underwriting   discounts   and
commissions)  payable  in  connection  with the sale of the  shares  of Series A
convertible  preferred  stock,  $.01  par  value  per  share  (the  "Convertible
Preferred  Stock")  and shares of common  stock,  $.001 par value per share (the
"Common  Stock"  and,  together  with  the  Convertible   Preferred  Stock,  the
"Securities") offered hereby are as follows:


SEC Registration fee......................................................
Printing and engraving expenses...........................................
Legal fees and expenses...................................................
Accounting fees and expenses..............................................
Blue Sky fees and expenses
  (including legal fees)..................................................
Transfer agent and registrar fees
  and expenses............................................................
Miscellaneous.............................................................
          Total...........................................................

The Registrant will bear all expenses shown above.

Item 14.  Indemnification of Directors and Officers.

            Article  EIGHTH  of  the   Registrant's   Restated   Certificate  of
Incorporation  provides that no director of the  Registrant  shall be personally
liable for any monetary  damages for any breach of fiduciary duty as a director,
except to the extent that the Delaware  General  Corporation  law  prohibits the
elimination  or  limitation  of liability  of directors  for breach of fiduciary
duty.

            Article  NINTH  of  the   Registrant's   Restated   Certificate   of
Incorporation provides that a director or officer of the Registrant (a) shall be
indemnified by the Registrant against all expenses (including  attorneys' fees),
judgments,  fines and amounts paid in settlement incurred in connection with any
litigation or other legal proceeding (other than an action by or in the right of
the  Registrant)  brought against him by virtue of his position as a director or
officer  of the  Registrant  if he  acted  in  good  faith  and in a  manner  he
reasonably  believed  to be in, or not  opposed  to, the best  interests  of the
Registrant,  and,  with respect to any  criminal  action or  proceeding,  had no
reasonable  cause  to  believe  his  conduct  was  unlawful  and  (b)  shall  be
indemnified by the Registrant against all expenses  (including  attorneys' fees)
and amounts paid in settlement  incurred in connection  with any action by or in
the right of the Registrant  brought  against him by virtue of his position as a
director or officer of the  Registrant if he acted in good faith and in a manner
he  reasonably  believed to be in, or not opposed to, the best  interests of the
Registrant,  except that no  indemnification  shall be made with  respect to any
matter as to which  such  person  shall have been  adjudged  to be liable to the
Registrant,  unless a court  determines that,  despite such  adjudication but in
view of all of the  circumstances,  he is  entitled to  indemnification  of such
expenses.  Notwithstanding  the  foregoing,  to the extent  that a  director  or
officer has been  successful,  on the merits or  otherwise,  including,  without
limitation,  the dismissal of an action without prejudice,  he is required to be
indemnified by the Registrant against all expenses  (including  attorneys' fees)
incurred in connection  therewith.  Expenses  shall be advanced to a director or
officer at his request, provided that he undertakes to repay the amount advanced
if it is ultimately  determined that he is not entitled to  indemnification  for
such expenses.

            Indemnification  is  required  to  be  made  unless  the  Registrant
determines that the applicable  standard of conduct required for indemnification
has not been met. In the event of a  determination  by the  Registrant  that the
director or officer did not meet the applicable standard of conduct required for
indemnification,  or if the Registrant fails to make an indemnification  payment
within 60 days after such payment is claimed by such

                                      II-1



<PAGE>

person,  such person is permitted  to petition the court to make an  independent
determination  as to whether  such person is entitled to  indemnification.  As a
condition  precedent  to the right of  indemnification,  the director or officer
must give the Registrant  notice of the action for which indemnity is sought and
the Registrant has the right to participate in such action or assume the defense
thereof.

            Article  NINTH  of  the   Registrant's   Restated   Certificate   of
Incorporation further provides that the indemnification  provided therein is not
exclusive,  and provides that in the event that the Delaware General Corporation
Law is amended to expand the indemnification  permitted to directors or officers
the Registrant must indemnify those persons to the full extent permitted by such
law as so amended.

            Section 145 of the Delaware General  Corporation Law provides that a
corporation has the power to indemnify a director, officer, employee or agent of
the  corporation  and  certain  other  persons  serving  at the  request  of the
corporation in related  capacities against amounts paid and expenses incurred in
connection  with an action or  proceeding  to which he is or is threatened to be
made a party by reason of such position, if such person shall have acted in good
faith and in a manner he reasonably believed to be in or not opposed to the best
interests of the corporation,  and, in any criminal  proceeding,  if such person
had no reasonable  cause to believe his conduct was unlawful;  provided that, in
the  case  of  actions  brought  by or in  the  right  of  the  corporation,  no
indemnification shall be made with respect to any matter as to which such person
shall have been adjudged to be liable to the corporation  unless and only to the
extent that the  adjudicating  court  determines  that such  indemnification  is
proper under the circumstances.

            Hybridon  is a  party  to  an  indemnification  agreement  with  Mr.
Grinstead.  Such agreement  provides that Mr.  Grinstead shall be indemnified by
the  Registrant  (a)  against  all  expenses  (as  defined  in  the  agreement),
judgments,  fines,  penalties  and  amounts  paid  in  settlement  actually  and
reasonably  incurred in  connection  with any legal  proceeding  (other than one
brought by or on behalf of the Registrant) if Mr.  Grinstead acted in good faith
and in a manner  which he  reasonably  believed to be in, or not opposed to, the
best interests of the Registrant,  and with respect to any criminal  proceeding,
had no reasonable cause to believe that his conduct was unlawful and (b) against
all expenses and amounts paid in settlement  actually and reasonably incurred in
connection with a legal proceeding  brought by or on behalf of the Registrant if
he acted in good faith and in a manner which he reasonably believed to be in, or
not  opposed  to,  the  best  interests  of  the  Registrant,   except  that  no
indemnification  shall be made in respect  of any  claim,  issue or matter as to
which Mr.  Grinstead  has been  adjudged to be liable.  If, with respect to such
proceedings, Mr. Grinstead is successful on the merits or otherwise, he shall be
reimbursed for all expenses.  Mr. Grinstead is required to provide notice to the
Registrant of any threatened or pending  litigation,  and the Registrant has the
right to participate in such action or assume the defense thereof.

            Hybridon  has obtained  directors  and  officers  insurance  for the
benefit of its directors and its officers.

Item 15.  Recent Sales of Unregistered Securities.

            In the  three  years  preceding  the  filing  of  this  registration
statement,  Hybridon has issued and sold its Common Stock,  warrants to purchase
its  Common  Stock,  Convertible  Subordinated  Notes and  Series A  Convertible
Preferred Stock, to certain  investors in transactions  that were not registered
under the Securities Act of 1933, as amended (the "Securities Act"):

Unregistered Offerings Pursuant to Section 4(2) Under the 1933 Act

            The securities issued in each of the following  transactions  (items
(1) through  (10)) were  offered and sold in reliance  upon the  exemption  from
registration  under Section 4(2) of the Securities Act,  relating to sales by an
issuer not involving a public  offering.  The  securities  issued in each of the
following  transactions  were  offered  and  sold  solely  to  persons  who were
"accredited investors" as that term is defined in Regulation D promulgated under
the Securities Act.

            (1) On January 20, 1997,  Hybridon  issued  25,000  shares of Common
Stock to an investment bank as compensation  under a financial advisory services
agreement dated that date.  These shares were offered and sold to an "accredited
investor"  (as that  term is  defined  in  Regulation  D  promulgated  under the
Securities

                                      II-2



<PAGE>

Act) in reliance upon the exemption from registration  under Section 4(2) of the
Securities  Act,  relating  to sales  by an  issuer  not  involving  any  public
offering.

            (2) On January 25, 1997,  Hybridon sold 1,650 shares of Common Stock
to one investor upon  exercise by such  investor of warrants to purchase  Common
Stock for an aggregate  purchase price of $9,075 . These shares were offered and
sold to an  "accredited  investor"  (as that term is  defined  in  Regulation  D
promulgated  under the  Securities  Act) in  reliance  upon the  exemption  from
registration  under Section 4(2) of the Securities Act,  relating to sales by an
issuer not involving any public offering.

            (3)  On  April  2,  1997,  Hybridon  issued  to an  investment  bank
$50,000,000  of its 9%  Notes.  These  9%  Notes  were  offered  and  sold to an
"accredited investor" (as that term is defined in Regulation D promulgated under
the  Securities  Act) in reliance upon the  exemption  from  registration  under
Section 4(2) of the Securities Act, relating to sales by an issuer not involving
any public offering.

            (4) On April 2, 1997, Hybridon issued to an investment bank warrants
to purchase  71,301 shares of Common Stock at an exercise  price of $35.0625 per
share. These warrants were offered and sold to an "accredited investor" (as that
term is  defined  in  Regulation  D  promulgated  under the  Securities  Act) in
reliance  upon  the  exemption  from  registration  under  Section  4(2)  of the
Securities  Act,  relating  to sales  by an  issuer  not  involving  any  public
offering.

            (5) On December 10, 1997,  Hybridon  issued to Dr. Paul Zamecnik,  a
Director of Hybridon, 50,000 shares of Common Stock of Hybridon.

            (6) On May 5, 1998, Hybridon accepted  $48,694,000  principal amount
of its 9% Notes  tendered to Hybridon in exchange for 510,505 shares of series A
preferred  stock (the  "Series A Preferred  Stock") and  warrants  (the "Class A
Warrants") to purchase  3,002,958  shares of common  stock,  par value $.001 per
share (the "Common Stock"),  of Hybridon (the "Exchange Offer").  As a result of
the Exchange Offer,  there is approximately $1.3 million principal amount of the
9% Notes outstanding.

            Pursuant to the Exchange Offer, which commenced on February 6, 1998,
all tendering  Noteholders  received per $1,000 principal amount of the 9% Notes
(including  accrued but unpaid interest on the 9% Notes) (i) 10 shares of Series
A Preferred Stock and (ii) Class A Warrants to purchase such number of shares of
Common  Stock equal to 25% of the number of shares of  Hybridon's  Common  Stock
into which the Series A Preferred  Stock issued to such  Noteholder  pursuant to
the Exchange Offer would be convertible.

            The   Convertible   Preferred  Stock  ranks,  as  to  dividends  and
liquidation  preference,  senior to  Hybridon's  Common Stock.  The  Convertible
Preferred  Stock issued in the Exchange  Offer and in the Regulation D Offering,
as defined below, as well as the Convertible  Preferred Stock that was issued as
a dividend on  September  30,  1998,  will be  convertible  into an aggregate of
15,088,200 shares of Common Stock, subject to adjustment, beginning May 5, 1999.

            The Class A Warrants will be  exercisable  commencing on May 5, 1999
for a period of four  years  thereafter  at $4.25  per  share of  Common  Stock,
subject to adjustment. The Class A Warrants are not subject to redemption at the
option of Hybridon under any circumstances.

            The Exchange  Offer was undertaken by Hybridon as part of Hybridon's
new business plan  contemplating  a  restructuring  of its capital  structure to
reduce debt service obligations, a significant reduction in its burn rate and an
infusion of additional equity capital.

            (7) On  May 5,  1998,  Hybridon  closed  a  private  placement  (the
"Regulation  D  Offering")  of (i) 114,285  shares of Series A Preferred  Stock,
which sold at $70 per share,  and (ii) class D warrants (the "Class D Warrants")
to purchase  672,273 shares of Hybridon's  Common Stock,  subject to adjustment,
for an aggregate amount of approximately $8 million.

            The Class D Warrants will be  exercisable  commencing on May 5, 1999
until May 4, 2003 at $2.40 per share of Common Stock, subject to adjustment.


                                      II-3



<PAGE>

            The net  proceeds to Hybridon  from the  Regulation  D Offering  are
presently used for general corporate  purposes,  primarily  research and product
development  activities,  including costs of preparing  investigational new drug
applications and conducting preclinical studies and clinical trials, the payment
of payroll  and other  accounts  payable  and for debt  service  required  under
Hybridon's debt  obligations.  The amounts actually expended by Hybridon and the
purposes of such  expenditures  may vary  significantly  depending upon numerous
factors,  including  the progress of  Hybridon's  research,  drug  discovery and
development  programs,  the results of preclinical  studies and clinical trials,
the timing of regulatory approvals,  sales of DNA products and reagents to third
parties  manufactured on a custom contract basis by the HSP Division and margins
on such  sales,  technological  advances,  determinations  as to the  commercial
potential of Hybridon's  compounds and the status of  competitive  products.  In
addition,  expenditures will also depend upon the establishment of collaborative
research arrangements with other companies,  the availability of other financing
and other factors. Under certain circumstances,  Hybridon may be required to use
net proceeds to repay indebtedness under the Bank Credit Facility.

            (8) On May 5, 1998,  Hybridon  closed a private  placement  of units
(the "Unit  Offering")  consisting of (i) 2,754,654  shares of Common Stock, and
(ii) class C warrants  (the "Class C Warrants")  to purchase  788,649  shares of
Common  Stock,   subject  to  adjustment,   which   securities  were  issued  in
consideration of the cancellation  (or reduction) of accounts  payable,  capital
lease and other obligations aggregating $5,509,308.

            The Class C Warrants are exercisable at $2.40 per share,  subject to
adjustment from time to time, until May 4, 2003.

            The Common Stock issued pursuant to the Unit Offering and the Common
Stock  underlying the Class C Warrants are subject to a "lock-up"  period ending
on May 5, 1999,  except to the extent such  securities  are sold or  transferred
pursuant  to a  Registration  Statement.  After  Hybridon  files a  Registration
Statement  under  the  Securities  Act,  75% of  each  holder's  Units  and  the
underlying  securities will be subject to an additional  "lock-up" for the first
three months  following the effective  date of the  Registration  Statement (the
"Effective  Date");  thereafter,  50% of such  securities  will be subject to an
additional  "lock-up"  until six months  following the Effective  Date;  and the
remaining 25% of such securities will be "locked-up" until nine months following
the Effective Date.

            (9) On May 5,  1998,  Hybridon  sold to Dr.  Paul  Zamecnik  100,000
shares of Common Stock and Class C Warrants to purchase  25,000 shares of Common
Stock, subject to adjustment, for a purchase price of $200,000.

            The net  proceeds  of this  offering  were used to  reduce  accounts
payable, capital lease and other obligations.

            (10) On May 5, 1998, Hybridon issued to certain suppliers a total of
362,500  shares of Common  Stock and Class C  Warrants  to  purchase  a total of
90,625 shares of Common Stock.  These  issuances  were in  consideration  of (i)
payment to  Hybridon  of a total of  $362.50,  the par value of all such  issued
Common  Stock,  and (ii) the  subsequent  furnishing  of  specified  services to
Hybridon by each  supplier.  The extent to which the  suppliers  have  completed
performing the specified services varies.

            (11) On December 12,  1998,  Hybridon  issued to Dr. Paul  Zamecnik
50,000 shares of Common Stock in  recognition  of Dr.  Zamecnik's  extraordinary
contribution to Hybridon.

            (12) On April  16,  1999,  Hybridon  issued  to  Pillar  Investments
Limited  300,000 shares of Common Stock in connection  with Pillar's  efforts in
assisting Hybridon with restructuring its balance sheet.

            (13) On May 1, 1999,  Hybridon  issued to Forum  Capital  Markey LLC
160,000 shares of Common Stock and warrants to purchase 173,333 shares of Common
Stock,  as a reinvestment by Forum of a $400,000 fee paid to Forum in connection
with the purchase of a bank loan to Hybridon.

            The Common  Stock  issued to Dr.  Paul  Zamecnik  and to the certain
suppliers and the Common Stock  underlying  the Class C Warrants  issued to such
persons are subject to a "lock-up"  period ending on May 5, 1999,  except to the
extent  such  securities  are sold or  transferred  pursuant  to a  Registration
Statement. After

                                      II-4



<PAGE>

Hybridon files a Registration  Statement  under the Securities  Act, 75% of each
holder's  Units and the underlying  securities  will be subject to an additional
"lock-up" for the first three months  following the Effective Date;  thereafter,
50% of such  securities  will be subject to an  additional  "lock-up"  until six
months  following the Effective  Date; and the remaining 25% of such  securities
will be "locked-up" until nine months following the Effective Date.

Unregistered Offerings Pursuant to Regulation S Under the Securities Act

            The  securities  issued  by  Hybridon  in the each of the  following
transactions   were  offered  and  sold  in  reliance  upon  an  exemption  from
registration  under Regulation S promulgated  under the Securities Act, relating
to sales by an issuer in offshore  transactions  (the "Regulation S Offerings").
The  securities  issued in each of the  following  Regulation  S Offerings  were
offered and sold solely to persons who were "accredited  investors" as that term
is defined in Regulation D promulgated under the Securities Act.

            (14) On January 15, 1998,  Hybridon commenced a private placement of
units (the "Units"), each Unit consisting of 14% Convertible  Subordinated Notes
Due 2007 (the "14% Notes") and  warrants  (the  "Equity  Warrants")  to purchase
shares of Hybridon's Common Stock (the "14% Note Offering").  The 14% Notes were
subject  to both  mandatory  and  optional  conversion  into  shares of series B
preferred stock,  under certain  circumstances  which, in turn, were convertible
into Common Stock (the "Series B Preferred Stock").

            On January 23, 1998, as part of the 14% Note Offering, Hybridon sold
$2,230,000 in principal amount of 14% Notes and Equity Warrants.

            On February 9, 1998, as part of the 14% Note Offering, Hybridon sold
$2,384,000 in principal amount of 14% Notes and Equity Warrants.

            On March 27, 1998, as part of the 14% Note  Offering,  Hybridon sold
$200,000 in principal amount of 14% Notes and Equity Warrants.

            On April 21, 1998, as part of the 14% Note  Offering,  Hybridon sold
$300,000 in principal amount of 14% Notes and Equity Warrants.

            On April 24, 1998, as part of the 14% Note  Offering,  Hybridon sold
$1,020,000 in principal amount of 14% Notes and Equity Warrants.

            In each of the above  closings,  the 14% Notes  were  issued at face
value.

            (15)  On May  5,  1998,  Hybridon  closed  a  private  placement  of
3,223,000  shares of Common Stock and class B warrants  (the "Class B Warrants")
to purchase  805,750 shares of Hybridon's  Common Stock,  subject to adjustment,
for aggregate gross proceeds of $6,446,000.

            The Class B Warrants are  exercisable  for a period of five years at
$2.40 per share of Common Stock, subject to adjustment from time to time.

            The Common  Stock issued in such  private  placement  and the Common
Stock  underlying  the Class B Warrants  issued in such  private  placement  are
subject to a "lock-up" for a period ending on May 5, 1999,  except to the extent
such  securities  are sold or transferred  pursuant to a Registration  Statement
filed by Hybridon under the Securities  Act. After Hybridon files a Registration
Statement under the Securities Act, 75% of each holder's Common Stock, including
the  Common  Stock  underlying  the  Class B  Warrants,  will be  subject  to an
additional  "lock-up" for the first three months  following the Effective  Date;
thereafter,  50% of such securities  will be subject to an additional  "lock-up"
until six months  following  the Effective  Date;  and the remaining 25% of such
securities will be "locked-up" until nine months following the Effective Date.

            (16) Hybridon has  exchanged all of the 14% Notes issued,  including
any right to interest thereon,  and all Equity Warrants issued together with the
14% Notes, for 3,157,322 shares of Common Stock and Class B Warrants to purchase
947,195 shares of Common Stock.


                                      II-5



<PAGE>

            The net proceeds to Hybridon  from the  Regulation  S Offerings  are
presently used for general corporate  purposes,  primarily  research and product
development  activities,  including costs of preparing  investigational new drug
applications and conducting preclinical studies and clinical trials, the payment
of payroll  and other  accounts  payable  and for debt  service  required  under
Hybridon's debt  obligations.  The amounts actually expended by Hybridon and the
purposes of such  expenditures  may vary  significantly  depending upon numerous
factors,  including  the progress of  Hybridon's  research,  drug  discovery and
development  programs,  the results of preclinical  studies and clinical trials,
the timing of regulatory approvals,  sales of DNA products and reagents to third
parties  manufactured on a custom contract basis by the HSP Division and margins
on such  sales,  technological  advances,  determinations  as to the  commercial
potential of Hybridon's  compounds and the status of  competitive  products.  In
addition,  expenditures will also depend upon the establishment of collaborative
research arrangements with other companies,  the availability of other financing
and other factors. Under certain circumstances,  Hybridon may be required to use
net proceeds to repay indebtedness under the Bank Credit Facility.


Item 16.  Exhibits and Financial Statement Schedules

(a)         Exhibits:

EXHIBIT INDEX

Exhibit No.           Description
- -----------           -----------

3.1(1)                Restated  Certificate of  Incorporation of the Registrant,
                      as amended.

3.2(2)                Amended and Restated By-Laws of the Registrant.

3.3(3)                Form of  Certificate  of Designation of Series A Preferred
                      Stock.

3.4(3)                Form of  Certificate  of Designation of Series B Preferred
                      Stock.

4.1(2)                Specimen Certificate for shares of Common Stock, $.001 par
                      value, of the Registrant.

4.2(4)                Indenture dated as of March 26, 1997 between Forum Capital
                      Markets LLC and the Registrant.

4.3(7)                Certificate of  Designation  of Series A Preferred  Stock,
                      par value $.01 per share, dated May 5, 1998.

4.4(7)                Class A Warrant Agreement dated May 5, 1998.

4.5(7)                Class B Warrant Agreement dated May 5, 1998.

4.6(7)                Class C Warrant Agreement dated May 5, 1998.

4.7(7)                Class D Warrant Agreement dated May 5, 1998.

+10.1(2)              License  Agreement  dated  February 21, 1990
                      and restaged as of September 8, 1993 between
                      the Registrant and the Worcester  Foundation
                      for Biomedical Research, Inc., as amended.

+10.2(2)              Patent License  Agreement dated September 21, 1995 between
                      the Registrant and National Institutes of Health.

+10.3(2)              Patent License Agreement  effective as of October 13, 1994
                      between the Registrant and McGill University.


                                      II-6



<PAGE>

+10.4(2)              License Agreement effective as of October 25, 1995 between
                      the Registrant and the General Hospital Corporation.

+10.5(2)              License Agreement dated as of October 30, 1995 between the
                      Registrant and Yoon S. Cho-Chung.

+10.6(2)              Collaborative Study Agreement effective as of December 30,
                      1992 between the Registrant and Medtronic, Inc.

+10.7(2)              System  Design  and  Procurement  Agreement  dated  as  of
                      December 16, 1994  between the  Registrant  and  Pharmacia
                      Biotech, Inc.

10.8(2)               Lease  dated March 10, 1994  between  the  Registrant  and
                      Laborer's Pension/Milford Investment Corporation for space
                      located at 155. Fortune Boulevard, Milford, Massachusetts,
                      including  Note  in  the  original   principal  amount  of
                      $750,000.

10.9(2)               Registration  Rights  Agreement  dated as of February  21,
                      1990 between the Registrant,  the Worcester Foundation for
                      Biomedical Research, Inc. and Paul C. Zamecnik.

10.10(2)              Registration  Rights  Agreement  dated as of June 25, 1990
                      between the Registrant and Nigel L. Webb.

10.11 (2)             Registration Rights Agreement dated as of February 6, 1992
                      between the Registrant and E. Andrews Grinstead, III.

10.12(2)              Registration Rights Agreement dated as of February 6, 1992
                      between the Registrant and Anthony J. Payne.

++10.13(2)            1990 Stock Option Plan, as amended.

++10.14(2)            1995 Stock Option Plan.

++10.15(2)            1995 Director Stock Plan.

++10.16(2)            1995 Employee Stock Purchase Plan.

10.17(2)              Form of  Warrant  originally  issued to Pillar  Investment
                      Limited  to  purchase  shares  of Common  Stock  issued as
                      placement  commissions  in  connection  with  the  sale of
                      shares  of  Series F  Convertible  Preferred  Stock and in
                      consideration of financial advisory service, as amended.

10.18(2)              Warrant  issued to Pillar S.A. to purchase  100,000 shares
                      of Common Stock dated as of March 1, 1994, as amended.

10.19(2)              Warrant  issued to Pillar S.A. to purchase  100,000 shares
                      of Common Stock dated as of March 1, 1995.

10.20(2)              Form of  Warrant  issued to Pillar  Investment  Limited to
                      purchase  shares  of  Common  Stock  issued  as  placement
                      commissions in connection  with the sale of Units pursuant
                      to the Series G Agreement.

++10.21(5)            Employment Agreement dated as of March 1, 1997 between the
                      Registrant and E. Andrews Grinstead, III.

10.22(2)              Indemnification  Agreement  dated as of  February  6, 1992
                      between the Registrant and E. Andrews Grinstead, III.


                                      II-7



<PAGE>

++10.23(6)            Employment  Agreement  dated  March 1,  1997  between  the
                      Registrant and Dr. Sudhir Agrawal.

++10.24(2)            Consulting Agreement dated as of February 21, 1990 between
                      the Registrant and Dr. Paul C. Zamecnik.

10.25(2)              Master Lease  Agreement  dated as of March 1, 1994 between
                      the Registrant and General Electric Capital Corporation.

+10.26(6)             Research,  Development  and License  Agreement dated as of
                      January 24, 1996 between the Registrant and G.D.  Searle &
                      Co.

+10.27(6)             Manufacturing and Supply Agreement dated as of January 24,
                      1996 between the Registrant and G.D. Searle & Co.

10.28(6)              Registration Rights Agreement dated as of January 24, 1996
                      between the Registrant and G.D. Searle & Co.

10.29(5)              Loan and Security  Agreement dated as of December 31, 1996
                      between the Registrant and Silicon Valley Bank.

10.30(7)              First Amendment to Loan and Security Agreement dated March
                      30, 1998 between Hybridon, Inc. and Silicon Valley Bank.

10.31(8)              Second Amendment to Loan and Security  Agreement dated May
                      19,  1998,   effective  as  of  April  30,  1998,  between
                      Hybridon, Inc. and Silicon Valley Bank.

10.32(9)              Third  Amendment  to Loan  and  Security  Agreement  dated
                      September  18, 1998  between  Hybridon,  Inc.  and Silicon
                      Valley Bank.

10.33(9)              Fourth  Amendment  to Loan and  Security  Agreement  dated
                      October  30,  1998,  effective  as of  September  29, 1998
                      between Hybridon, Inc. and Silicon Valley Bank.

10.34(12)             Fifth  Amendment  to Loan  and  Security  Agreement  dated
                      December 4, 1998 between Hybridon, Inc. and Silicon Valley
                      Bank.

10.35(5)              Warrant issued to Silicon  Valley Bank to purchase  65,000
                      shares of Common Stock dated as of December 31, 1996.

10.36(5)              Registration  Rights  Agreement  dated as of December  31,
                      1996 between the Registrant and Silicon Valley Bank.

+10.37(5)             Supply and Sales  Agreement  dated as of September 1, 1996
                      between the Registrant and P.E. Applied Biosystems.

10.38(2)              Registration  Rights  Agreement dated as of March 26, 1997
                      between Forum Capital Markets LLC and the Registrant.

10.39(2)              Warrant Agreement dated as of March 26, 1997 between Forum
                      Capital Markets LLC and the Registrant.

+10.40(6)             Amendment  No. 1 to License  Agreement,  dated as February
                      21, 1990 and  restated  as of  September  8, 1993,  by and
                      between the Worcester  Foundation for Biomedical Research,
                      Inc. and the Registrant, dated as of November 26, 1996.


                                      II-8



<PAGE>

10.41(10)             Letter Agreement dated May 12, 1997 between the Registrant
                      and Pillar S.A. amending the Consulting Agreement dated as
                      of March 1, 1994 between the Registrant and Pillar S.A.

10.42(10)             Amendment  dated July 15, 1997 to the Series G Convertible
                      Preferred Stock and Warrant Purchase Agreement dated as of
                      September  9,  1994  among  the   Registrant  and  certain
                      purchasers, as amended.

10.43(1)              Consent  Agreement  dated January 15, 1998 between Silicon
                      Valley  Bank and the  Registrant  relating  to the Silicon
                      Agreement.

10.44(11)             Letter Agreement  between the Registrant and Forum Capital
                      Markets LLC and Pecks  Management  Partners  Ltd.  for the
                      purchase of the Loan and Security  Agreement  with Silicon
                      Valley Bank.

10.45(7)              Financial Advisory Agreement between Registrant and Pillar
                      Investments Ltd. dated May 5, 1998.

10.46(7)              Placement Agency Agreement  between  Registrant and Pillar
                      Investments Ltd. dated as of January 15, 1998.

+++10.47(12)          Licensing  Agreement  dated  March 12, 1999 by and between
                      Hybridon, Inc. and Integrated DNA Technologies, Inc.

21.1(2)               Subsidiaries of the Registrant.

23.1                  Consent of Arthur Andersen LLP.

23.2                  Consent of McDonnell Boehnen Hulbert & Berghoff.

27.1                  Financial Data Schedule  [EDGAR] - Year Ended December 31,
                      1998

 ----------------------------------

(1)                   Incorporated by reference to Exhibits to the  Registrant's
                      Annual Report on Form 10-K for the year ended December 31,
                      1997.

(2)                   Incorporated by reference to Exhibits to the  Registrant's
                      Registration Statement on Form S-1 (File No. 33-99024).

(3)                   Incorporated  by  reference  to  Exhibit  9(a)(1)  to  the
                      Registrant's Schedule 13E-4 dated February 6, 1998.

(4)                   Incorporated by reference to Exhibits to the  Registrant's
                      Current Report on Form 8-K dated April 2, 1997.

(5)                   Incorporated by reference to Exhibits to the  Registrant's
                      Annual Report on Form 10-K for the year ended December 31,
                      1996.

(6)                   Incorporated by reference to Exhibits to the  Registrant's
                      Annual Report on Form 10-K for the year ended December 31,
                      1995.

(7)                   Incorporated by reference to Exhibits to the  Registrant's
                      Quarterly  Report on Form 10-Q for the period  ended March
                      31, 1998.

(8)                   Incorporated by reference to Exhibits to the  Registrant's
                      Quarterly  Report on Form 10-Q for the  period  ended June
                      30, 1998.


                                      II-9



<PAGE>

(9)                   Incorporated by reference to Exhibits to the  Registrant's
                      Quarterly  Report  on  Form  10-Q  for  the  period  ended
                      September 30, 1998.

(10)                  Incorporated by reference to Exhibits to the  Registrant's
                      Quarterly  Report on Form 10-Q for the  period  ended June
                      30, 1997.

(11)                  Incorporated by reference to Exhibits to the  Registrant's
                      Registration Statement on Form S-1 (File No. 333-69649).

(12)                  Incorporated by reference to Exhibits to the  Registrant's
                      Annual  Report on Form 10- K for the year  ended  December
                      31, 1998

+                     Confidential  treatment  granted as to  certain  portions,
                      which portions are omitted and filed  separately  with the
                      Commission.

++                    Management  contract or  compensatory  plan or arrangement
                      required to be filed as an Exhibit to the Annual Report on
                      Form 10-K for the year ended December 31, 1997.

+++                   Confidential  treatment  requested as to certain portions,
                      which portions are omitted and filed  separately  with the
                      Commission.




                                      II-10



<PAGE>

Item 17.  Undertakings.

            Insofar  as  indemnification   for  liabilities  arising  under  the
Securities Act may be permitted to directors,  officers and controlling  persons
of the  Registrant  pursuant  to  provisions  described  in  Item 14  above,  or
otherwise, the Registrant has been advised that in the opinion of the Securities
and  Exchange  Commission  such  indemnification  is  against  public  policy as
expressed in the Securities Act and is, therefore,  unenforceable.  In the event
that a claim  for  indemnification  against  such  liabilities  (other  than the
payment by the Registrant of expenses incurred or paid by a director, officer or
controlling  person of the Registrant in the  successful  defense of any action,
suit or proceeding) is asserted by such director,  officer or controlling person
in connection with the securities being registered,  the Registrant will, unless
in the  opinion  of its  counsel  the matter  has been  settled  by  controlling
precedent,  submit to a court of appropriate  jurisdiction  the question whether
such  indemnification  by it is  against  public  policy  as  expressed  in  the
Securities Act and will be governed by the final adjudication of such issue.

The undersigned registrant hereby undertakes:

(1)     To file,  during any period in which  offers or sales are being made,  a
        post-effective amendment to this Registration Statement:

(2)     To include any Prospectus required by Section 10(a)(3) of the Securities
        Act of 1933;

(3)     To  reflect  in the  Prospectus  any facts or events  arising  after the
        effective  date  of the  Registration  Statement  (or  the  most  recent
        post-effective   amendment  thereof)  which,   individually  or  in  the
        aggregate,  represent a fundamental  change in the information set forth
        in  the  Registration  Statement.  Notwithstanding  the  foregoing,  any
        increase  or  decrease  in volume of  Securities  offered  (if the total
        dollar  value of  securities  offered  would not  exceed  that which was
        registered)  and any deviation from the low or high end of the estimated
        maximum  offering range may be reflected in the form of prospectus filed
        with the Commission  pursuant to Rule 424(b) if, in the  aggregate,  the
        changes in volume and price  represent no more than 20 percent change in
        the maximum  aggregate  offering price set forth in the  "Calculation of
        Registration Fee" table in the effective registration statement.

(4)     To  include  any  material  information  with  respect  to the  plan  of
        distribution not previously  disclosed in the Registration  Statement or
        any material change to such information in the Registration Statement.

(5)     That, for the purpose of determining  any liability under the Securities
        Act,  each  such  post-effective  amendment  shall be deemed to be a new
        Registration  Statement relating to the Securities offered therein,  and
        the offering of such  securities  at that time shall be deemed to be the
        initial bona fide offering thereof.

(6)     To remove from  registration by means of a post-effective  amendment any
        of  the  Securities   being   registered  which  remain  unsold  at  the
        termination of the offering.


                                      II-11



<PAGE>

                                   SIGNATURES

Pursuant to the  requirements  of the  Securities  Act of 1933, as amended,  the
Registrant  has duly  caused  this  Registration  Statement  to be signed on its
behalf by the undersigned, thereunto duly authorized, on June 28, 1999.

                                       HYBRIDON,  INC.


                                       By: /s/  E. ANDREWS GRINSTEAD, III
                                       ----------------------------------
                                       E. Andrews Grinstead, III
                                       Chairman, Chief Executive Officer


Pursuant to the  requirements  of the Securities  Act of 1933, as amended,  this
Registration  Statement  has  been  signed  by  the  following  persons  in  the
capacities and on the dates indicated.


      Signatures                         Title(s)                  Date
      ----------                         --------                  ----

/s/ E. ANDREWS GRINSTEAD, III       Chairman, Chief Executive   June 28, 1999
- -------------------------------
E. Andrews Grinstead, III           Officer and Director

*                                   Senior Vice President and   June 28, 1999
- -------------------------------
Dr. Sudhir Agrawal                  Director

*                                   Director                    June 28, 1999
- -------------------------------
 Dr. James B. Wyngaarden

*                                   Director                    June 28, 1999
- -------------------------------
 Mr. Nasser Menhall

*                                   Director                    June 28, 1999
- -------------------------------
 Dr. Paul C. Zamecnik

*                                   Director                    June 28, 1999
- -------------------------------
 Mr. Youssef El-Zein

                                    Director                    June 28, 1999
 Mr. Arthur W. Berry

*                                   Director                    June 28, 1999
- -------------------------------
Mr. Harold L. Purkey

                                    Director                    June 28, 1999
Mr. Camile Chebeir

                                    Director                    June 28, 1999
Mr. H.F. Powell


*   By: /s/ E. ANDREWS GRINSTEAD, III
       ------------------------------
            E. Andrews Grinstead, III
            Attorney-in-fact




<PAGE>

EXHIBIT INDEX

Exhibit No.         Description
- -----------         -----------

3.1(1)            Restated Certificate of Incorporation of the Registrant,  as
                  amended.

3.2(2)            Amended and Restated By-Laws of the Registrant.

3.3(3)            Form of  Certificate  of  Designation  of Series A Preferred
                  Stock.

3.4(3)            Form of  Certificate  of  Designation  of Series B Preferred
                  Stock.

4.1(2)            Specimen  Certificate for shares of Common Stock,  $.001 par
                  value, of the Registrant.

4.2(4)            Indenture  dated as of March 26, 1997 between  Forum Capital
                  Markets LLC and the Registrant.

4.3(7)            Certificate of Designation of Series A Preferred  Stock, par
                  value $.01 per share, dated May 5, 1998.

4.4(7)            Class A Warrant Agreement dated May 5, 1998.

4.5(7)            Class B Warrant Agreement dated May 5, 1998.

4.6(7)            Class C Warrant Agreement dated May 5, 1998.

4.7(7)            Class D Warrant Agreement dated May 5, 1998.

+10.1(2)          License Agreement dated February 21, 1990 and restaged as of
                  September 8, 1993 between the  Registrant  and the Worcester
                  Foundation for Biomedical Research, Inc., as amended.

+10.2(2)          Patent License  Agreement  dated  September 21, 1995 between
                  the Registrant and National Institutes of Health.

+10.3(2)          Patent  License  Agreement  effective as of October 13, 1994
                  between the Registrant and McGill University.

+10.4(2)          License  Agreement  effective as of October 25, 1995 between
                  the Registrant and the General Hospital Corporation.

+10.5(2)          License  Agreement  dated as of October 30, 1995 between the
                  Registrant and Yoon S. Cho-Chung.

+10.6(2)          Collaborative  Study Agreement  effective as of December 30,
                  1992 between the Registrant and Medtronic, Inc.

+10.7(2)          System Design and Procurement Agreement dated as of December
                  16, 1994 between the Registrant and Pharmacia Biotech, Inc.

10.8(2)           Lease  dated  March 10,  1994  between  the  Registrant  and
                  Laborer's  Pension/Milford  Investment Corporation for space
                  located at 155. Fortune Boulevard,  Milford,  Massachusetts,
                  including Note in the original principal amount of $750,000.

10.9(2)           Registration  Rights Agreement dated as of February 21, 1990
                  between  the  Registrant,   the  Worcester   Foundation  for
                  Biomedical Research, Inc. and Paul C. Zamecnik.

10.10(2)          Registration  Rights  Agreement  dated as of June  25,  1990
                  between the Registrant and Nigel L. Webb.

10.11(2)          Registration  Rights  Agreement dated as of February 6, 1992
                  between the Registrant and E. Andrews Grinstead, III.




<PAGE>

10.12(2)          Registration  Rights  Agreement dated as of February 6, 1992
                  between the Registrant and Anthony J. Payne.

++10.13(2)        1990 Stock Option Plan, as amended.

++10.14(2)        1995 Stock Option Plan.

++10.15(2)        1995 Director Stock Plan.

++10.16(2)        1995 Employee Stock Purchase Plan.

10.17(2)          Form of  Warrant  originally  issued  to  Pillar  Investment
                  Limited  to  purchase  shares  of  Common  Stock  issued  as
                  placement  commissions in connection with the sale of shares
                  of Series F Convertible Preferred Stock and in consideration
                  of financial advisory service, as amended.

10.18(2)          Warrant issued to Pillar S.A. to purchase  100,000 shares of
                  Common Stock dated as of March 1, 1994, as amended.

10.19(2)          Warrant issued to Pillar S.A. to purchase  100,000 shares of
                  Common Stock dated as of March 1, 1995.

10.20(2)          Form of  Warrant  issued to  Pillar  Investment  Limited  to
                  purchase   shares  of  Common   Stock  issued  as  placement
                  commissions in connection with the sale of Units pursuant to
                  the Series G Agreement.

++10.21(5)        Employment  Agreement  dated as of March 1, 1997 between the
                  Registrant and E. Andrews Grinstead, III.

10.22(2)          Indemnification  Agreement  dated  as of  February  6,  1992
                  between the Registrant and E. Andrews Grinstead, III.

++10.23(6)        Employment   Agreement  dated  March  1,  1997  between  the
                  Registrant and Dr. Sudhir Agrawal.

++10.24(2)        Consulting  Agreement  dated as of February 21, 1990 between
                  the Registrant and Dr. Paul C. Zamecnik.

10.25(2)          Master Lease Agreement dated as of March 1, 1994 between the
                  Registrant and General Electric Capital Corporation.

+10.26(6)         Research,  Development  and  License  Agreement  dated as of
                  January 24, 1996 between the  Registrant  and G.D.  Searle &
                  Co.

+10.27(6)         Manufacturing  and Supply  Agreement dated as of January 24,
                  1996 between the Registrant and G.D. Searle & Co.

10.28(6)          Registration  Rights  Agreement dated as of January 24, 1996
                  between the Registrant and G.D. Searle & Co.

10.29(5)          Loan and  Security  Agreement  dated as of December 31, 1996
                  between the Registrant and Silicon Valley Bank.

10.30(7)          First  Amendment to Loan and Security  Agreement dated March
                  30, 1998 between Hybridon, Inc. and Silicon Valley Bank.

10.31(8)          Second  Amendment to Loan and Security  Agreement  dated May
                  19, 1998,  effective as of April 30, 1998, between Hybridon,
                  Inc. and Silicon Valley Bank.

10.32(9)          Third  Amendment  to  Loan  and  Security   Agreement  dated
                  September 18, 1998 between Hybridon, Inc. and Silicon Valley
                  Bank.

10.33(9)          Fourth  Amendment  to  Loan  and  Security  Agreement  dated
                  October 30, 1998, effective as of September 29, 1998 between
                  Hybridon, Inc. and Silicon Valley Bank.


                                    II-14



<PAGE>

10.34(12)         Fifth  Amendment  to  Loan  and  Security   Agreement  dated
                  December 4, 1998 between  Hybridon,  Inc. and Silicon Valley
                  Bank.

10.35(5)          Warrant  issued to Silicon  Valley Bank to  purchase  65,000
                  shares of Common Stock dated as of December 31, 1996.

10.36(5)          Registration  Rights Agreement dated as of December 31, 1996
                  between the Registrant and Silicon Valley Bank.

+10.37(5)         Supply and Sales  Agreement  dated as of  September  1, 1996
                  between the Registrant and P.E. Applied Biosystems.

10.38(2)          Registration  Rights  Agreement  dated as of March 26,  1997
                  between Forum Capital Markets LLC and the Registrant.

10.39(2)          Warrant  Agreement  dated as of March 26, 1997 between Forum
                  Capital Markets LLC and the Registrant.

+10.40(6)         Amendment No. 1 to License Agreement,  dated as February 21,
                  1990 and restated as of  September  8, 1993,  by and between
                  the Worcester Foundation for Biomedical  Research,  Inc. and
                  the Registrant, dated as of November 26, 1996.

10.41(10)         Letter  Agreement  dated May 12, 1997 between the Registrant
                  and Pillar S.A.  amending the Consulting  Agreement dated as
                  of March 1, 1994 between the Registrant and Pillar S.A.

10.42(10)         Amendment  dated July 15,  1997 to the Series G  Convertible
                  Preferred Stock and Warrant  Purchase  Agreement dated as of
                  September   9,  1994  among  the   Registrant   and  certain
                  purchasers, as amended.

10.43(1)          Consent  Agreement  dated  January 15, 1998 between  Silicon
                  Valley  Bank  and the  Registrant  relating  to the  Silicon
                  Agreement.

10.44(11)         Letter  Agreement  between the  Registrant and Forum Capital
                  Markets  LLC and  Pecks  Management  Partners  Ltd.  for the
                  purchase of the Loan and  Security  Agreement  with  Silicon
                  Valley Bank.

10.45(7)          Financial  Advisory  Agreement between Registrant and Pillar
                  Investments Ltd. dated May 5, 1998.

10.46(7)          Placement  Agency  Agreement  between  Registrant and Pillar
                  Investments Ltd. dated as of January 15, 1998.

+++10.47(12)      Licensing  Agreement  dated  March 12,  1999 by and  between
                  Hybridon, Inc. and Integrated DNA Technologies, Inc.

21.1(2)           Subsidiaries of the Registrant.

23.1              Consent of Arthur Andersen LLP.

23.2              Consent of McDonnell Boehnen Hulbert & Berghoff.

27.1              Financial Data Schedule [EDGAR] - Year Ended December 31, 1998

- ------------------------------

(1)               Incorporated  by reference  to Exhibits to the  Registrant's
                  Annual  Report on Form 10-K for the year ended  December 31,
                  1997.

(2)               Incorporated  by reference  to Exhibits to the  Registrant's
                  Registration Statement on Form S-1 (File No. 33-99024).

(3)               Incorporated   by  reference  to  Exhibit   9(a)(1)  to  the
                  Registrant's Schedule 13E-4 dated February 6, 1998.


                                    II-15



<PAGE>

(4)               Incorporated  by reference  to Exhibits to the  Registrant's
                  Current Report on Form 8-K dated April 2, 1997.

(5)               Incorporated  by reference  to Exhibits to the  Registrant's
                  Annual  Report on Form 10-K for the year ended  December 31,
                  1996.

(6)               Incorporated  by reference  to Exhibits to the  Registrant's
                  Annual  Report on Form 10-K for the year ended  December 31,
                  1995.

(7)               Incorporated  by reference  to Exhibits to the  Registrant's
                  Quarterly Report on Form 10-Q for the period ended March 31,
                  1998.

(8)               Incorporated  by reference  to Exhibits to the  Registrant's
                  Quarterly  Report on Form 10-Q for the period ended June 30,
                  1998.

(9)               Incorporated  by reference  to Exhibits to the  Registrant's
                  Quarterly Report on Form 10-Q for the period ended September
                  30, 1998.

(10)              Incorporated  by reference  to Exhibits to the  Registrant's
                  Quarterly  Report on Form 10-Q for the period ended June 30,
                  1997.

(11)              Incorporated  by reference  to Exhibits to the  Registrant's
                  Registration Statement on Form S-1 (File No. 333-69649).

(12)              Incorporated  by reference  to Exhibits to the  Registrant's
                  Annual Report on Form 10- K for the year ended  December 31,
                  1998

+                 Confidential treatment granted as to certain portions, which
                  portions   are  omitted  and  filed   separately   with  the
                  Commission.

++                Management  contract  or  compensatory  plan or  arrangement
                  required  to be filed as an Exhibit to the Annual  Report on
                  Form 10-K for the year ended December 31, 1997.

+++               Confidential  treatment  requested  as to certain  portions,
                  which  portions  are omitted and filed  separately  with the
                  Commission.



                                      II-16



                                                                  EXHIBIT 23.1

                    CONSENT OF INDEPENDENT PUBLIC ACCOUNTANTS


            As independent public  accountants,  we hereby consent to the use of
our report  (and to all  references  to our Firm)  included in or made a part of
this Registration Statement.


                                                 /s/ ARTHUR ANDERSEN LLP

Boston, Massachusetts
July 2, 1999


                                      II-17



                                                                  EXHIBIT 23.2

                 CONSENT OF MCDONNELL BOEHNEN HULBERT & BERGHOFF

              [Letterhead of McDonnell Boehnen Hulbert & Berghoff]

                                                             July 2, 1999

Hybridon, Inc.
155 Fortune Blvd.
Milford, MA 01757

                  RE:  Hybridon, Inc -- Registration Statement on Form S-1

Dear Sirs:

            McDonnell,  Boehnen,  Hulbert  &  Berghoff  hereby  consents  to the
reference to our firm under the section "The  Company - Patents,  Trade  Secrets
and Licenses"  included in this Registration  Statement on Form S-1 of Hybridon,
Inc., and any pre-effective or post-effective amendments thereto.

                                                     Very truly yours,


                                                     /s/ John J. McDonnell
                                                     ---------------------
                                                     John J. McDonnell



                                      II-18




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