REPLIGEN CORP
S-3, 2000-01-28
BIOLOGICAL PRODUCTS, (NO DIAGNOSTIC SUBSTANCES)
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<PAGE>


                  AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON , 2000
                                                    REGISTRATION NO. ___________

================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                              WASHINGTON, DC 20549
                    ----------------------------------------
                                    FORM S-3
                             REGISTRATION STATEMENT
                                      UNDER
                           THE SECURITIES ACT OF 1933
                    ----------------------------------------
                              REPLIGEN CORPORATION
             (Exact name of Registrant as specified in its charter)
                          -----------------------------

<TABLE>
<S>                                              <C>                       <C>
            DELAWARE                             2836                      04-2729386
   (State or other jurisdiction       (Primary Standard Industrial      (I.R.S. Employer
 of incorporation or organization)     Classification Code Number)    Identification Number)
</TABLE>

                                117 FOURTH AVENUE
                                NEEDHAM, MA 02494
                                 (781) 449-9560
               (Address, including zip code, and telephone number,
        including area code, of Registrant's principal executive offices)
                    ----------------------------------------
                                WALTER C. HERLIHY
                      PRESIDENT AND CHIEF EXECUTIVE OFFICER
                              REPLIGEN CORPORATION
                                117 FOURTH AVENUE
                                NEEDHAM, MA 02494
                                 (781) 449-9560
            (Name, address, including zip code, and telephone number,
                   including area code, of agent for service)

                    ----------------------------------------
  COPIES OF ALL COMMUNICATIONS, INCLUDING ALL COMMUNICATIONS SENT TO THE AGENT
                        FOR SERVICE, SHOULD BE SENT TO:

                          LAWRENCE S. WITTENBERG, ESQ.
                         Testa, Hurwitz & Thibeault, LLP
                                High Street Tower
                                 125 High Street
                           Boston, Massachusetts 02110
                                 (617) 248-7000

                         CALCULATION OF REGISTRATION FEE
<TABLE>
<CAPTION>
- ------------------------------------------------------------------------------------------------------------------------------------
  TITLE OF EACH CLASS OF SECURITIES TO BE   AMOUNT TO BE      PROPOSED MAXIMUM          PROPOSED MAXIMUM             AMOUNT OF
              REGISTERED                     REGISTERED    OFFERING PRICE PER SHARE*  AGGREGATE OFFERING PRICE*    REGISTRATION FEE*
- ------------------------------------------------------------------------------------------------------------------------------------
<S>                                          <C>                 <C>                      <C>                         <C>
Common Stock, $0.01 par value per share
(issuable upon exercise of the Exchange      1,653,250            $8.00                   $13,226,000                 $3,491.66
Warrants)
- ------------------------------------------------------------------------------------------------------------------------------------
Common Stock, $0.01 par value per share
(issuable upon exercise of the Modified        353,800           $14.00                   $4,953,200                  $1,307.64
Original Warrants)
- ------------------------------------------------------------------------------------------------------------------------------------
</TABLE>

*  Pursuant to Rule 457(g) under the Securities Act of 1933, the proposed
   maximum offering price per share, the proposed maximum aggregate offering
   price and the amount of the registration fee with respect to shares to be
   offered pursuant to Exchange Warrants and the Modified Original Warrants have
   been computed in accordance with clause (1) thereof.

APPROXIMATE DATE OF COMMENCEMENT OF PROPOSED SALE TO THE PUBLIC: As soon as
practicable after this Registration Statement becomes effective.

     If the only securities being registered on this form are being offered
pursuant to dividend or interest reinvestment plans, please check the following
box.  / /

     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, other than securities offered only in connection with dividend or interest
reinvestment plans, 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.  / /

     REPLIGEN HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE OR DATES AS
MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL REPLIGEN 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 SECTION 8(a), MAY DETERMINE.
================================================================================
<PAGE>

[begin red herring]

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 securities, and it is not soliciting offers to buy these securities in
any state where the offer or sale is not permitted.

[end red herring]

                  SUBJECT TO COMPLETION, DATED January 28, 2000

                              REPLIGEN CORPORATION

                                2,007,050 SHARES

                                  COMMON STOCK

     This Prospectus relates to 2,007,050 shares of common stock, $.01 par
value per share, of Repligen Corporation issuable upon exercise of warrants
previously issued by Repligen. Repligen's common stock is traded on the
Nasdaq National Market under the symbol "RGEN." The last reported sale price
of the common stock of Repligen on the Nasdaq National Market on January 26,
2000 was $5.00 per share.

     Repligen is offering 2,007,050 shares of its common stock issuable upon the
exercise of certain warrants originally issued by it in February 1992 in a
financing transaction to purchasers of units and to the sales agent for the
units and the agent's affiliates. Each unit consisted of a Class A limited
partnership interest in Repligen Clinical Partners, L.P. and a warrant to
purchase 2,900 shares of common stock of Repligen. Of the warrants originally
issued as part of the units, certain warrants were exchanged in March 1994 and
later modified in March 1995 to reduce the exercise price of such warrants (the
"Exchange Warrants"), and the remaining warrants which were not exchanged in
March 1994 were later modified in March 1995 to reduce the exercise price of
such warrants (the "Modified Original Warrants"). The terms and conditions of
the offer and sale of the shares issuable upon exercise of the warrants,
including the price per share, are governed by the provisions of the warrants as
described herein under the caption "Description of Warrants." Repligen will
receive all proceeds from the exercise of the warrants.

<TABLE>
<CAPTION>
- -----------------------------------------------------------------------------------------------------------
                    Shares of common
                     stock issuable   Per Share      Aggregate    Underwriting                  Net
                    upon exercise of  Price to      Proceeds to   Discounts and    Other      Proceeds to
                      warrants         Public         Company      Commissions    Expenses     Company
- -----------------------------------------------------------------------------------------------------------

<S>                   <C>             <C>           <C>               <C>         <C>         <C>
Modified Original     353,800         $9.00-        $3,184,200-       -0-         $30,000     $3,154,200-
Warrants(1)                           $14.00        $4,953,200                                $4,923,200
- -----------------------------------------------------------------------------------------------------------

Exchange                              $2.50/$3.50-  $5,216,289-       -0-         $30,000     $5,186,289-
Warrants(2)         1,653,250         $8.00         $13,226,000                               $13,196,000
- -----------------------------------------------------------------------------------------------------------
</TABLE>

     (1)The Modified Original Warrants are exercisable at $9.00 per share until
the date that is 90 days after Repligen notifies the holder of the Modified
Original Warrants that the Nasdaq National Market closing price per share of the
common stock of Repligen equaled or exceeded $18.00 per share for any 20 out of
30 consecutive trading days, at which time the Modified Original Warrants will
become exercisable at $14.00 per share.

     (2)The exercise price of the first 1,000 shares issuable upon exercise of
the Exchange Warrants is $2.50, and the remaining 1,900 shares issuable upon
exercise of the Exchange Warrants are exercisable at $3.50. The exercise price
for all of the shares subject to the Exchange Warrants will increase to $8.00
per share on the date 90 days after Repligen notifies warrantholders that the
Nasdaq National Market closing price per share of the common stock equaled or
exceeded $12.00 per share for 20 out of 30 consecutive trading days.

                  INVESTING IN THE COMMON STOCK INVOLVES RISKS.
                     SEE "RISK FACTORS" BEGINNING ON PAGE 3.

NEITHER THE SECURITIES AND EXCHANGE COMMISSION NOR ANY STATE SECURITIES
COMMISSION HAS APPROVED OR DISAPPROVED 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 January ___, 2000.
<PAGE>

YOU SHOULD RELY ONLY ON THE INFORMATION CONTAINED IN THIS PROSPECTUS. WE HAVE
NOT AUTHORIZED ANYONE TO PROVIDE YOU WITH INFORMATION DIFFERENT FROM THAT
CONTAINED IN THIS PROSPECTUS. THE INFORMATION CONTAINED IN THIS PROSPECTUS IS
ACCURATE ONLY AS OF THE DATE OF THIS PROSPECTUS, REGARDLESS OF THE TIME OF
DELIVERY OF THIS PROSPECTUS OR OF ANY SALE OF THE COMMON STOCK.

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

                                TABLE OF CONTENTS

<TABLE>
<CAPTION>
                                                          PAGE

<S>                                                       <C>
Repligen...............................................    2
Risk Factors...........................................    3
Legal Proceedings......................................    6
Use of Proceeds........................................    6
Dilution...............................................    6
Description of the Warrants............................    7
Legal Matters..........................................   11
Experts................................................   11
Where You Can Find More Information....................   11
</TABLE>

                            ------------------------
<PAGE>

                                    REPLIGEN

     We develop new drugs for neurological disease, organ transplantation and
cancer. To expand our drug development program, on March 9, 1999, we acquired
the exclusive rights to patent applications for the use of secretin in the
treatment of autism. Autism is a developmental disorder characterized by poor
communicative and social skills, repetitive and restricted behaviors and in some
patients, gastrointestinal problems and irregular sleep patterns. Secretin is a
hormone produced in the small intestine which regulates the function of the
pancreas as part of the process of digestion. A form of secretin derived from
pigs is approved by the FDA for use in diagnosing problems with pancreatic
function. Recent anecdotal reports indicate that secretin may have beneficial
effects in autism, including improvements in sleep, digestive function,
communicative and social behavior. Following media reports of the potential
benefits of secretin, more than 2,000 autistic children have been treated with
the pig-derived hormone. We intend to manufacture a human, synthetic form of
secretin and evaluate it in FDA approved clinical trials in order to confirm the
benefits of secretin in treating autism and to determine the optimal dosing
schedule. There are currently no drugs approved by the FDA for the treatment of
autism.

     In October 1999, Repligen licensed commercialization rights to two
diagnostic secretin products from ChiRhoClin Inc., a private company. These
products have been evaluated in clinical trials for the diagnosis of pancreatic
dysfunction and gastrinoma. A New Drug Application was filed with the FDA in May
1999 seeking approval to market synthetic porcine secretin for these
applications. ChiRhoClin has also conducted clinical studies for these
diagnostic indications with a human form of secretin which it intends to submit
to the FDA in 2000. Under terms of the agreement, Repligen made an upfront
payment upon execution of the agreement and, if the FDA approves the New Drug
Applications, the agreement obligates Repligen to pay ChiRhoClin future
milestones and royalties.

     We are also developing a product named "CTLA4-Ig," which has been shown
to suppress unwanted immune responses in animal models of organ transplants
and autoimmune diseases, such as lupus or multiple sclerosis, in which the
immune system mistakenly attacks the body. Our product candidate is a
derivative of a natural protein whose role is to turn-off an immune response.
In animal models of organ transplantation and autoimmune diseases, CTLA4-Ig
has been shown to block the rejection of a transplanted organ or the effects
of the autoimmune disease. Initial clinical testing of CTLA4-Ig has been
carried out in patients receiving a bone marrow transplant, which is a
potential cure for several diseases of the immune system, including leukemia,
myeloma, lymphoma and sickle cell anemia. Despite the clinical success of
bone marrow transplants, a significant number of patients experience a severe
and potentially life-threatening complication known as Graft Versus Host
Disease, in which the newly transplanted immune system attacks the host
(i.e., the patient). In June 1999, results from a Phase 1 clinical trial
reported that treatment of bone marrow from a family member with Repligen's
CTLA4-Ig prevented Graft Versus Host Disease in eight of eleven transplant
patients. In September 1999, we signed a Clinical Trial Agreement with the
National Cancer Institute to further evaluate CTLA4-Ig in a Phase 2 trial in
bone marrow transplantation for leukemia. Repligen has filed patent
applications related to compositions of matter and methods of use of CTLA4-Ig
including bone marrow transplantation. Certain patents have been issued to
Bristol-Meyers Squibb Corporation relating to the use and manufacturing of
CTLA4-Ig. We believe that through one of our licenses we have rights

                                      -2-
<PAGE>

of one or more of these patents and that the patents issued to Bristol-Meyers
Squibb do not extend to our use of CTLA4-Ig in bone marrow transplantation.
For more information on our patent litigation, please see "Legal Proceeding".

     We develop, manufacture and market products for the production of
therapeutic antibodies. We currently market a line of products for the
purification of antibodies based on a naturally occurring protein, Protein A,
which can specifically bind to antibodies. Repligen owns composition of matter
patents for recombinant Protein A in the United States and in Europe. In
December 1998, we entered into a ten year agreement to supply recombinant
Protein A to Amersham Pharmacia Biotech, a leading supplier to the
biopharmaceutical market.

     Repligen's executive offices are located at 117 Fourth Avenue, Needham,
Massachusetts 02494, and Repligen's telephone number is (781) 449-9560.

                                  RISK FACTORS

     YOU SHOULD CAREFULLY CONSIDER THE RISKS DESCRIBED BELOW BEFORE MAKING AN
INVESTMENT DECISION. ADDITIONAL RISKS AND UNCERTAINTIES THAT WE ARE UNAWARE OF
OR THAT WE CURRENTLY DEEM IMMATERIAL ALSO MAY BECOME IMPORTANT FACTORS THAT
AFFECT REPLIGEN.

     IF ANY OF THE FOLLOWING RISKS OCCUR, OUR BUSINESS, FINANCIAL CONDITION OR
RESULTS OF OPERATIONS COULD BE MATERIALLY HARMED. IN THAT CASE THE TRADING PRICE
OF OUR COMMON STOCK COULD DECLINE, AND YOU MAY LOSE ALL OR PART OF YOUR
INVESTMENT.

     THIS PROSPECTUS ALSO CONTAINS FORWARD LOOKING STATEMENTS THAT INVOLVE RISKS
AND UNCERTAINTIES. OUR ACTUAL RESULTS COULD DIFFER MATERIALLY FROM THOSE
ANTICIPATED IN THESE FORWARD LOOKING STATEMENTS AS A RESULT OF CERTAIN FACTORS,
INCLUDING THE RISKS FACED BY US DESCRIBED BELOW AND ELSEWHERE IN THIS
PROSPECTUS.

IF WE DO NOT OBTAIN ADDITIONAL CAPITAL, WE WILL BE UNABLE TO DEVELOP OR DISCOVER
NEW DRUGS.

     We need additional long-term financing to develop our drug development
programs through the clinical trial process as required by the FDA and our
bioprocessing products business. We also need additional long-term financing to
support future operations and capital expenditures, including capital for
additional personnel and facilities. If we spend more money than currently
expected for our drug development programs and our bioprocessing products
business, we will need to raise additional capital by selling debt or equity
securities, by entering into strategic relationships or through other
arrangements. We may be unable to raise any additional amounts on reasonable
terms when they are needed due to the volatile nature of the biotechnology
marketplace. If we are unable to raise this additional capital, we may have to
delay or postpone critical clinical studies or abandon other development
programs.

IF WE ARE UNABLE TO CONTINUE TO HIRE AND RETAIN SKILLED TECHNICAL AND SCIENTIFIC
PERSONNEL, THEN WE WILL HAVE TROUBLE DEVELOPING PRODUCTS.

     Our success depends largely upon the continued service of our management
and scientific staff and our ability to attract, retain and motivate highly
skilled scientific, management and marketing personnel. Potential employees with
an expertise in the field of biochemistry, regulatory


                                      -3-
<PAGE>

affairs and/or clinical development of new drug and biopharmaceutical
manufacturing are not generally available in the market and are difficult to
attract and retain. We also face significant competition for such personnel from
other companies, research and academic institutions, government and other
organizations who have superior funding and resources to be able to attract such
personnel. The loss of key personnel or our inability to hire and retain
personnel who have technical and scientific backgrounds could materially
adversely affect our product development efforts and our business.

WE COMPETE WITH LARGER, BETTER FINANCED AND MORE MATURE PHARMACEUTICAL AND
BIOTECHNOLOGY COMPANIES WHO ARE CAPABLE OF DEVELOPING NEW APPROACHES THAT COULD
MAKE OUR PRODUCTS AND TECHNOLOGY OBSOLETE.

     The market for therapeutic and bioprocessing products is intensely
competitive, rapidly evolving and subject to rapid technological change.
Pharmaceutical and mature biotechnology companies have substantially greater
financial, manufacturing, marketing, research and development resources than we
have. New approaches to the treatment of our targeted diseases by these
competitors may make our products and technologies obsolete or noncompetitive.

IF WE ARE UNABLE TO OBTAIN AND MAINTAIN PATENTS FOR OUR PRODUCTS, WE WILL NOT BE
ABLE TO SUCCEED COMMERCIALLY.

     We must obtain patent and trade secret protection for our products and
processes in order to protect them from unauthorized use and to produce a
financial return consistent with the significant time and expense required to
bring our products to market. Our success will depend, in part, on our ability
to:

     o obtain patent protection for our products and manufacturing processes;

     o preserve our trade secrets; and

     o operate without infringing the proprietary rights of third parties.

     There can be no assurance that any patent applications relating to our
products will be filed in the future or that any currently pending applications
will issue on a timely basis, if ever. Since patent applications in the United
States are maintained in secrecy until patents issue and since publication of
discoveries in the scientific or patent literature often lag behind actual
discoveries, we cannot be certain that we were the first to make the inventions
covered by each of our pending patent applications or that we were the first to
file patent applications for such inventions. Even if patents are issued, the
degree of protection afforded by such patents will depend upon the:

     o scope of the patent claims;

     o validity and enforceability of the claims obtained in such patents; and

     o our willingness and financial ability to enforce and/or defend them.

     The patent position of biotechnology and pharmaceutical firms is often
highly uncertain and usually involves complex legal and scientific questions.
Moreover, no consistent policy has emerged in the United States and in many
other countries regarding the breadth of claims allowed in biotechnology
patents. Patents which may be granted to us in certain foreign


                                      -4-

<PAGE>

countries may be subject to opposition proceedings brought by third parties or
result in suits by Repligen which may be costly and result in adverse
consequences for Repligen.

     If our competitors prepare and file patent applications in the United
States that claim technology also claimed by us, we may be required to
participate in interference proceedings declared by the U.S. Patent and
Trademark Office to determine priority of invention, which would result in
substantial costs to us.

     In addition, patents blocking our manufacture, use or sale of our products
could be issued to third parties in the United States or foreign countries. The
issuance of blocking patents or an adverse outcome in an interference or
opposition proceeding, could subject us to significant liabilities to third
parties and require us to license disputed rights from third parties on
unfavorable terms, if at all, or cease using the technology.

WE MAY NOT BE YEAR 2000 COMPLIANT, AND AS A RESULT, MAY FACE, AND BE LIABLE FOR,
DATA CORRUPTION, COMPUTER FAILURE AND DISRUPTION OF OPERATIONS.

     Many existing computer systems and software products do not properly
recognize dates after December 31, 1999. This "Year 2000" problem could result
in miscalculations, data corruption, system failures or disruptions of
operations. We are subject to potential Year 2000 problems affecting our
computers' systems, our internal systems and the systems of our vendors and
scientific collaborators, any of which could have a material adverse affect on
our business operating results and financial conditions.

     Based on our assessments to date, we believe that our internal systems are
substantially Year 2000 compliant although there can be no assurance that Year
2000 errors or defects will not be discovered in our internal software systems
and, if such errors or defects are discovered, there can be no assurance that
the costs of making such systems Year 2000 compliant will not be material.

     Year 2000 errors or defects in the internal systems maintained by our
vendors of clinical trial collaborators could require us to incur significant
delays in our product development programs and unanticipated expenses to remedy
any problems or replace affected vendors. To date, no Y2K problems have been
detected by us and we are not aware of any problems with our suppliers or
collaborators.


                                      -5-
<PAGE>

                                LEGAL PROCEEDINGS

     On July 17, 1998, Repligen filed a complaint against Bristol-Meyers Squibb
Corporation ("BMS") at the United States District Court for the District of
Massachusetts in Boston, Massachusetts seeking correction of inventorship of
certain United States patents which claim compositions and methods of use for
CTLA4 as well as unspecified monetary damages. A correction of inventorship
would result in the University of Michigan being designated as a co-assignee on
any corrected BMS patent. Repligen would then have rights to such technology
pursuant to a 1992 License Agreement with the University of Michigan, a 1995
Asset Acquisition Agreement with Genetics Institute, and other related
agreements. On July 13, 1999, the court dismissed the complaint without
prejudice citing a lack of legal standing of Repligen to bring such a complaint.
We believe that the court's finding on standing was in error. The court did not
rule on the validity of Repligen's inventorship claim. Repligen continues to
believe that the University of Michigan is a rightful co-assignee of the
aforesaid BMS patents and we intend to continue to pursue the correction of
inventorship. Repligen's failure to obtain shared ownership rights in the
patents may restrict Repligen's ability to commercialize CTLA4-Ig.

                                 USE OF PROCEEDS

     If all of the warrants are exercised, the net proceeds to Repligen from the
exercise of the warrants (after deducting estimated expenses in connection with
this offering) would range from $8,390,489 to $18,169,200, depending on when the
warrants are exercised. Repligen intends to use the net proceeds, if any, from
the sale of the common stock offered hereby for research and development,
including ongoing development of Repligen's technologies, pre-clinical and
clinical testing, and other costs associated with Repligen's product development
programs, for capital expenditures, and for working capital and other general
purposes. Repligen is offering the shares of common stock issuable upon exercise
of the warrants and filing this registration statement related thereto in order
to fulfill its contractual obligations to the warrantholders to maintain an
effective registration statement during the time that the warrants are
outstanding and exercisable.

                                    DILUTION

     The net tangible book value of Repligen, as of September 30, 1999, was
$12.4 million, or $.57 per share of common stock. Net tangible book value per
share represents the amount of total tangible assets less total liabilities
divided by the number of shares of common stock outstanding at that date.

     After giving effect to the sale by Repligen of the 2,007,050 shares of
common stock being offered hereby (assuming the exercise of all of the warrants)
at an assumed exercise price of $2.50 and $3.50 per share for the Exchange
Warrants and $9.00 for the Modified Original Warrants, our pro forma net
tangible book value as of September 30, 1999, would have been $20.8 million or
$.87 per share. This represents an immediate increase in pro forma net tangible
book value of $.30 per share to existing stockholders and an immediate dilution
of $1.63 to $8.13 per share to new investors. The following table illustrates
this per share dilution:


                                      -6-
<PAGE>

<TABLE>
<S>                                                                                <C>         <C>
Assumed exercise price per share:
   Exchange Warrants exercisable for 570,086 shares........................                    $  2.50
   Exchange Warrants exercisable for 1,083,164 shares......................                       3.50
   Modified Original Warrants exercisable for 353,800 shares...............                       9.00
   Net tangible book value per share at September 30, 1999.................        $  .57
   Increase per share attributable to new investors........................           .30
                                                                                   ------
Pro forma net tangible book value per share after this offering............                        .87
                                                                                               -------
Dilution per share to new investors

   Holders of Exchange Warrants exercisable for $2.50 per share............                    $  1.63
   Holders of Exchange Warrants exercisable for $3.50 per share............                    $  2.63
   Holders of Modified Original Warrants exercisable for $9.00 per share...                    $  8.13
</TABLE>

     Alternatively, if the warrants are adjusted such that the exercise price
becomes $8.00 per share for the Exchange Warrants and $14.00 per share for
the Modified Original Warrants, and after giving effect to the sale by
Repligen of the shares of common stock being offered hereby (assuming the
exercise of all the warrants) at such exercise prices, our pro forma net
tangible book value as of September 30, 1999, would have been $30.6 million
or $1.28 per share. This represents an immediate increase in pro forma net
tangible book value of $.71 per share to existing stockholders and an
immediate dilution of $6.72 to $12.72 per share to new investors. The
following table illustrates this per share dilution:

<TABLE>
<S>                                                                                     <C>      <C>
Assumed exercise price per share:
   Exchange Warrants exercisable for 1,653,250 shares...........................                 $  8.00
   Modified Original Warrants exercisable for 353,800 shares....................                   14.00
   Net tangible book value per share at September 30, 1999......................        $  .57
   Increase per share attributable to new investors.............................           .71
                                                                                        ------
Pro forma net tangible book value per share after this offering.................                    1.28
                                                                                                 -------
Dilution per share to new investors
   Holders of Exchange Warrants exercisable for $8.00 per share.................                 $  6.72
   Holders of Modified Original Warrants exercisable for $14.00 per share.......                 $ 12.72
</TABLE>

                DESCRIPTION OF THE WARRANTS/PLAN OF DISTRIBUTION

     In connection with the initial capitalization of Repligen Clinical
Partners, L.P. in February, 1992, Repligen issued to certain purchasers and the
sales agent related thereto and the agent's affiliates, units in Repligen
Clinical Partners. Each unit consisted of a Class A limited partnership interest
in Repligen Clinical Partners and a warrant (collectively, the "Original
Warrants") to purchase 2,900 shares of common stock of Repligen.

     In March 1994, Repligen offered to the holders of Original Warrants the
opportunity to exchange their Original Warrants for newly issued warrants (the
"Exchange Warrants"). Each holder of an Original Warrant was free to accept or
reject the exchange offer. Many


                                      -7-
<PAGE>

warrantholders holding warrants to purchase shares of Repligen common stock
accepted the exchange offer. Some warrantholders were, however, ineligible to
participate in this exchange offer due to their failure to make payments on
promissory notes (the "Promissory Notes") which were originally issued by
certain of the warrantholders in favor of Repligen Clinical Partners to pay for
the units. Some warrantholders who were eligible to participate in the exchange
offer rejected the exchange offer and thus continued to hold Original Warrants.

     In connection with the March 1994 exchange of Original Warrants for
Exchange Warrants, the exercise price under the Exchange Warrants was reduced to
$9.00 (from $22.73) per share, subject to increase to $14.00 per share on the
date 90 days after Repligen notified the warrantholders holding the Exchange
Warrants that the closing price of Repligen common stock equaled or exceeded
$18.00 per share for 20 out of 30 consecutive trading days. The exercise period
under the Exchange Warrants was also extended to March 31, 2000. The holders of
Exchange Warrants also accepted a reduction in certain royalty rates that each
warrantholder would have received as a limited partner of Repligen Clinical
Partners, such royalties arising out of any sales of the drug (rPF4) being
developed by Repligen Clinical Partners. Acceptance of the Exchange Warrants
also resulted in pro rata reductions in certain other royalties which were
payable to such warrantholders as limited partners of Repligen Clinical
Partners.

     In March 1995, Repligen subsequently offered to the warrantholders an
opportunity to modify the Original Warrants and the Exchange Warrants that
remained outstanding. With respect to each holder of an outstanding Original
Warrant or Exchange Warrant who was required to make and did make the fourth
installment payment to Repligen Clinical Partners pursuant to the Promissory
Notes, such holders (along with the other warrantholders who did not have
outstanding Promissory Notes) were free to accept or reject the proposed
modifications.

     With respect to the Original Warrants, the modifications provided that the
terms of the Original Warrants would be modified (the "Modified Original
Warrants") so as to be identical to those of the Exchange Warrants issued in the
1994 exchange offer (i.e., an extension of the exercise period by one year to
March 31, 2000 and a per share exercise price reduction from $22.73 to $9.00,
subject to escalation to $14.00 per share 90 days after Repligen notified
holders thereof that the closing price of Repligen common stock equaled or
exceeded $18.00 per share for 20 out of 30 consecutive trading days).

     With respect to the Exchange Warrants being modified in March 1995, the
exercise price of the first 1,000 shares of Repligen common stock issuable upon
exercise thereof was reduced from $9.00 per share to $2.50 per share, and the
exercise price for the remaining 1,900 shares of Repligen common stock issuable
upon exercise thereof was reduced from $9.00 per share to $3.50 per share. The
exercise price of the modified Exchange Warrants was subject to increase to
$8.00 per share on the date 90 days after Repligen notified holders thereof that
the closing price of Repligen common stock equaled or exceeded $12.00 per share
for 20 out of 30 consecutive trading days. Lastly, Repligen further modified the
Exchange Warrants by extending the expiration period for the Exchange Warrants
by one year, to March 31, 2001. The proposed modifications did not include
further reductions in any royalty rates payable by Repligen. The difference in
treatment between the Original Warrants and Exchange Warrants in the March 1995
modification was due to the fact that holders of Original Warrants were those
warrantholders


                                      -8-
<PAGE>

who did not accept the 1994 exchange offer and consequently had a right to
receive higher royalty rates than holders of Exchange Warrants.

     The exercise price and the number of shares issuable upon exercise of each
warrant will be appropriately adjusted in the event of stock splits, stock
combinations, stock dividends, reclassifications or rights offerings involving
common stock or distributions of certain subscription rights, warrants or
evidences of indebtedness to holders of common stock. Repligen will not issue
fractional shares upon exercise of the warrants. Instead Repligen will pay to
the holder a cash adjustment equal to the amount of fractional shares based on
the closing price of common stock as reported on the Nasdaq National Market on
the date of exercise.

     With certain exceptions, in case of any reclassification or capital
reorganization, or in case of any consolidation or merger of Repligen or any
sale, lease, transfer or conveyance of all or substantially all of the assets of
Repligen, each holder of a warrant hereunder shall have the right, upon
subsequent exercise of such warrant, to purchase the kind and amount of shares
of stock or other securities and property receivable upon such reclassification,
capital reorganization, consolidation, merger, sale, lease, transfer or
conveyance by a holder of the number of shares of common stock that might have
been received upon the exercise of such warrant immediately prior to such event,
and the exercise price of such warrants will be appropriately adjusted. The
warrants do not confer upon the warrantholders any rights as stockholders of
Repligen.

     As of December 1, 1999, Modified Original Warrants and Exchange Warrants to
purchase 353,800 shares of Repligen common stock were issued and outstanding and
such warrants will expire on March 31, 2000. As of December 1, 1999, modified
Exchange Warrants to purchase 1,653,250 shares of Repligen common stock were
issued and outstanding and such warrants will expire on March 31, 2001.

     Repligen had previously filed a registration statement in April 1994 to
register the issuance of common stock upon exercise of the warrants issued as
part of the units issued by Repligen Clinical Partners. Because that
registration statement became inactive (due to in part because the exercise
price of the warrants was significantly higher than the fair market value of the
underlying common stock), Repligen is filing this registration statement to
register the issuance of shares of common stock upon exercise of the Modified
Original Warrants and the modified Exchange Warrants in compliance with its
contractual obligations to the warrantholders to maintain an effective
registration statement during the period the warrants are outstanding and
exercisable.

WARRANT EXERCISE PROCEDURE

     Repligen has not engaged any underwriter, broker or dealer in connection
with the issuance of the shares to the warrantholders and will pay no
commissions thereon. In order to effectively exercise the warrants and purchase
the shares issuable upon exercise thereof, a warrantholder must comply with the
procedure described below. Failure to follow this procedure may result in either
a delay in the warrantholder's receipt of a stock certificate or an ineffective
exercise and return to the warrantholder of the materials submitted for
exercise.

     The warrantholder must use the "Purchase Form" attached to the end of the
certificate representing the warrant ("the Warrant Certificate"). The
warrantholder must specify the number of


                                      -9-
<PAGE>

shares being purchased and the exercise payment being remitted. The
warrantholder must sign the Purchase Form where indicated and the signature must
conform with the manner in which the Warrant Certificate is held as appears on
the warrant register maintained by Repligen (the "Warrant Register").

     Each holder of a Warrant Certificate must deliver the completed Purchase
Form, and cash or a check drawn to the order of "Repligen Corporation" for the
exact amount of the exercise price for the shares being purchased to the
Director of Finance, Repligen Corporation, 117 Fourth Avenue Needham, MA 02494.
In addition, to satisfy Internal Revenue Service requirements, the exercising
warrantholder should provide Repligen with his or her social security number or
taxpayer identification number. Also, an exercising warrantholder should provide
Repligen with the address to which he, she or it wants the share certificate
delivered. Each warrantholder should direct any questions concerning exercise of
the warrants to the Director of Finance at the address listed above.

     Upon receipt of the Warrant Certificate, including the completed Purchase
Form, appropriate payment for the shares being purchased, and the exercising
warrantholder's social security or taxpayer identification number, Repligen will
instruct its transfer agent to issue a share certificate in the warrantholder's
name (or other name if so designated) for the shares being purchased. The
transfer agent will send the share certificate to the exercising warrantholder
at the address indicated at the time of exercise; otherwise the transfer agent
will send the share certificate to the address listed on the Warrant Register.
Unless otherwise agreed to by Repligen, Repligen will send all share
certificates by first class mail with an estimated delivery time of about four
weeks from the date of Repligen's receipt of the exercise material.

     Upon receipt of the Warrant Certificate, including the completed Purchase
Form, appropriate payment for the shares being purchased, and the exercising
warrantholder's social security or taxpayer identification number, the
warrantholder will become the holder of record of those shares being purchased
and shall have all of the rights of a stockholder, including the right to vote
the shares.

PARTIAL EXERCISE

     A warrantholder may exercise his or her Warrant Certificate for only part
of the shares the Warrant Certificate represents. If a warrantholder exercises
only a part of the shares represented by a Warrant Certificate, then in addition
to the share certificate for the exercised shares, he or she will receive a new
Warrant Certificate, dated the date of his or her original Warrant Certificate,
representing those shares still subject to exercise.

     EquiServe, 150 Royall Street, Canton, MA 02021, is the transfer agent for
the shares of common stock of Repligen.


                                      -10-
<PAGE>

                                  LEGAL MATTERS

     The validity of the shares of common stock of Repligen offered hereby will
be passed upon for Repligen by Testa, Hurwitz & Thibeault, LLP, Boston,
Massachusetts.

                                     EXPERTS

     The financial statements incorporated by reference in this prospectus
and elsewhere in the registration statement to the extent and for the periods
indicated in their reports have been audited by Arthur Andersen LLP,
independent public accountants, as indicated in their reports with respect
thereto, and are included herein in reliance upon the authority of said firm
as experts in auditing and accounting.

                       WHERE YOU CAN FIND MORE INFORMATION

     Repligen files annual, quarterly and special reports, proxy statements and
other information with the SEC. You may read and copy any document we file with
the SEC at the SEC's public reference room at 450 Fifth Street, N.W.,
Washington, D.C. 20549. Please call the SEC at 1-800-SEC-0330 for further
information on operation of the public reference room. Our SEC filings are also
available to the public from the SEC's website at "http://www.sec.gov." Our
website is located at "http://www.repligen.com." Information contained on our
website is not part of this prospectus.

     The SEC allows Repligen to "incorporate by reference" the information we
file with them, which means that we can disclose important information to you by
referring you to those documents. The information incorporated by reference is
considered to be part of this prospectus, and information that we file later
with the SEC will automatically update and supersede this information. Repligen
incorporates by reference the documents listed below and any future filings we
will make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Exchange
Act (File No. 000-14656):

1.   Annual report on Form 10-K for the year ended March 31, 1999;

2.   Repligen's proxy statement, filed on July 29, 1999, for the 1999 annual
     meeting of shareholders;

3.   Quarterly reports on Form 10-Q for the quarters ended September 30, 1999
     and June 30, 1999;

4.   Current reports on Form 8-K filed October 6, 1999, March 24, 1999, as
     amended by Form 8-K/A filed June 15, 1999 and current report on Form 8-K
     filed May 17, 1999; and

5.   The "Description of Registrant's Securities to be Registered" contained in
     Repligen's registration statement filed on Form 8-A, dated May 28, 1986.

     You may request a copy of these filings, at no cost, by writing or
telephoning our Chief Financial Officer at the following address: Repligen
Corporation, 117 Fourth Avenue Needham, MA 02494 (781) 449-9560.


                                      -11-
<PAGE>

     This prospectus is part of a registration statement we filed with the SEC.
You should rely only on the information or representations provided in this
prospectus. We have authorized no one to provide you with different information.
We are not making an offer of these securities in any state where the offer is
not permitted. You should not assume that the information in this prospectus is
accurate as of any date other than the date on the front of the document.


                                      -12-
<PAGE>

                                     PART II

                     INFORMATION NOT REQUIRED IN PROSPECTUS

ITEM 14.  OTHER EXPENSES OF ISSUANCE AND DISTRIBUTION.

                  The following table sets forth an estimate (other than with
respect to the Registration Fee) of the expenses expected to be incurred in
connection with the issuance and distribution of the securities being
registered, other than underwriting discounts and commissions:

<TABLE>
<S>                                                       <C>
Registration Fee -- Securities and Exchange Commission....$   4,799.30
Blue Sky Fees and Expenses................................$   1,000.00
Accounting Fees and Expenses..............................$   3,000.00
Legal Fees and Expenses...................................$  10,000.00
Transfer Agent fees and expenses..........................$   5,000.00
Miscellaneous.............................................$   6,200.70
                                                          ------------
                  TOTAL...................................$  30,000.00
                                                          ============
</TABLE>

     Repligen will bear all expenses shown above.

ITEM 15.  INDEMNIFICATION OF DIRECTORS AND OFFICERS.

     The Delaware General Corporation Law, Article Seventh of Repligen's
Restated Certificate of Incorporation, as amended, and Article V of Repligen's
By-laws provide for indemnification of Repligen's directors and officers for
liabilities and expenses that they may incur in such capacities. In general,
directors and officers are indemnified with respect to actions taken in good
faith in a manner reasonably believed to be in, or not opposed to, the best
interests of Repligen, and with respect to any criminal action or proceeding,
actions that the indemnitee had no reasonable cause to believe were unlawful.

     Repligen maintains directors and officers liability insurance for the
benefit of its directors and certain of its officers.


                                      II-1
<PAGE>

ITEM 16.          EXHIBITS.

     The following exhibits, required by Item 601 of Regulation S-K, are filed
as a part of this Registration Statement. Exhibit numbers, where applicable, in
the left column correspond to those of Item 601 of Regulation S-K.

<TABLE>
<CAPTION>
EXHIBIT NO.       ITEM AND REFERENCE

<S>           <C>
4.1               Form of Modified Limited Partner Warrant, dated as of
                  February 28, 1992 (referred to as the Modified Original
                  Warrant) (filed as Exhibit 4.3 to Repligen Corporation's
                  Annual Report on Form 10-K for the year ended March 31, 1998
                  and incorporated herein by reference).

+4.2              Form of Amended, Modified and Restated Warrant (referred to as
                  the Exchange Warrant).

+5            --  Legal Opinion of Testa, Hurwitz & Thibeault, LLP

+23.1         --  Consent of Arthur Andersen LLP

23.2          --  Consent of Testa, Hurwitz & Thibeault, LLP (included in
                  Exhibit 5)

+24           --  Power of Attorney
</TABLE>

- ------------------
+ filed herewith

ITEM 17.          UNDERTAKINGS.

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:

          (i) To include any prospectus required by Section 10(a)(3) of the
Securities Act of 1933;

          (ii) 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,
represents 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 and 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;

          (iii) 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.


                                      II-2
<PAGE>

     (2) That, for the purpose of determining any liability under the Securities
Act of 1933, 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.

     (3) 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.

     The undersigned registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act of 1933, each filing of the
registrant's annual report pursuant to Section 13(a) or 15(d) of the Securities
Exchange Act of 1934 (and, where applicable, each filing of an employee benefit
plan's annual report pursuant to Section 15(d) of the Securities Exchange Act of
1934) that is incorporated by reference in the registration statement 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.

     Insofar as indemnification for liabilities arising under the Securities Act
of 1933 may be permitted to directors, officers and controlling persons of the
registrant pursuant to provisions described in Item 15 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 of 1933 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 of 1933 and will be governed by the final adjudication of such
issue.


                                      II-3
<PAGE>

                                   SIGNATURES

     Pursuant to the requirements of the Securities Act of 1933, the registrant
certifies that it has reasonable grounds to believe that it meets all of the
requirements for filing on Form S-3 and has duly caused this Registration
Statement on Form S-3 to be signed on its behalf by the undersigned, thereunto
duly authorized in the Town of Needham, Commonwealth of Massachusetts on January
28, 2000.

                              Repligen Corporation

                              By: /s/ Walter C. Herlihy
                                  ------------------------------------
                                  Walter C. Herlihy
                                  President and Chief Executive
                                    Officer



                                POWER OF ATTORNEY

     Each person whose signature appears below on this Registration Statement
hereby constitutes and appoints Walter C. Herlihy with full power to act as his
true and lawful attorney-in-fact and agent, with full power of substitution and
resubstitution, for him and in his name, place and stead, in any and all
capacities (until revoked in writing) to sign any and all amendments (including
post-effective amendments and amendments thereto) to this Registration Statement
on Form S-3 of Repligen Corporation, and to file the same, with all exhibits
thereto, and other documents in connection therewith, with the Securities and
Exchange Commission, granting unto said attorney-in-fact and agents full power
and authority to do and perform each and every act and thing requisite and
necessary fully to all intents and purposes as he might or could do in person
thereby ratifying and confirming all that said attorney-in-fact and agents or
any of them, or their or his substitute or substitutes, may lawfully do or cause
to be done by virtue hereof.

     Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement on Form S-3 has been signed below by the following
persons on behalf of the Registrant and in the capacities and on the dates
indicated.

<TABLE>
<CAPTION>
NAME                                      CAPACITY                                    DATE

<S>                                       <C>                                         <C>
/s/ Walter C. Herlihy                     President and Chief Executive               January 28, 2000
- --------------------------------------    Officer, Chief Financial Officer
Walter C. Herlihy                         and Director (principal executive,
                                          financial and accounting officer)

/s/ Alexander Rich, M.D.                  Co-Chairman of the Board of                 January 28, 2000
- --------------------------------------    Directors
Alexander Rich, M.D.


                                      II-4
<PAGE>

/s/ Paul Schimmel, Ph.D.                  Co-Chairman of the Board of                 January 28, 2000
- --------------------------------------    Directors
Paul Schimmel, Ph.D.

/s/ Robert J. Hennessey                   Director                                    January 28, 2000
- --------------------------------------
Robert J. Hennessey

/s/ G. William Miller                     Director                                    January 28, 2000
- --------------------------------------
G. William Miller
</TABLE>


                                      II-5
<PAGE>

                                INDEX TO EXHIBITS

<TABLE>
<CAPTION>
EXHIBIT NO.       ITEM AND REFERENCE

<S>           <C>
4.1               Form of Modified Limited Partner Warrant, dated as of February
                  28, 1992 (referred to as the Modified Original Warrant) (filed
                  as Exhibit 4.3 to Repligen Corporation's Annual Report on Form
                  10-K for the year ended March 31, 1998 and incorporated herein
                  by reference).

+4.2              Form of Amended, Modified and Restated Warrant (referred to as
                  the Exchange Warrant).

+5            --  Legal Opinion of Testa, Hurwitz & Thibeault, LLP

+23.1         --  Consent of Arthur Andersen LLP

23.2          --  Consent of Testa, Hurwitz & Thibeault, LLP (included in
                  Exhibit 5)

+24           --  Power of Attorney
</TABLE>

- -----------------
+ filed herewith


                                      II-6

<PAGE>

             AMENDED, MODIFIED AND RESTATED LIMITED PARTNER WARRANT

                              REPLIGEN CORPORATION

               WARRANT FOR THE PURCHASE OF SHARES OF COMMON STOCK

     Name and address of
     Registered Holder:                      {NAME}
                                             {ADDRESS 1}
                                             {ADDRESS 2}
                                             {CITY, STATE, COUNTRY}
                                             {ZIP/POSTAL CODE}

     No. RCP{# OF WARRANT}               {# OF SHARES} Shares

     IN CONSIDERATION OF the exchange and subsequent modification of the warrant
to purchase common stock of Repligen Corporation, a Delaware corporation (the
"Company"), dated February 28, 1992 (the "Limited Partner Warrant") held by the
initial registered holder thereof (the "Initial Holder"), and the Initial
Holder's agreement to amend the Amended and Restated Purchase Agreement dated as
of February 28, 1992 (the "Purchase Agreement"), to reduce certain royalties
payable by the company thereunder, and for value received, the Company hereby
grants the rights herein specified and certifies that the Initial Holder or any
registered assignee of the Initial Holder (each of the Initial Holder and any
such registered assignee being hereinafter referred to as the "Holder") is
entitled, subject to the conditions and upon the terms of this Warrant, to
purchase from the Company, at any time or from time to time during the Exercise
Period (as defined in Section 1 hereof), {# OF SHARES} shares of Common Stock
(as defined in Section 1 hereof). The number of shares of Common Stock to be
received upon the exercise of this Warrant and the Exercise Price are subject to
adjustment from time to time as hereinafter set forth. This Warrant is one of
the Warrants issued pursuant to the Company's offer (the "Exchange Offer") to
reduce the exercise price and to extend the exercise period and which was
subsequently modified (the "Modification Offer") to further reduce the exercise
price and to further extend the exercise period under the Limited Partner
Warrants, the Class B Warrant and the Fund Warrant (the "Existing Warrants")
issued in connection with the initial sale of limited partnership interests in
Repligen Clinical Partners, L.P., a Delaware limited partnership (the
"Partnership") and replaces the Amended and Restated Limited Partner Warrants
and Limited Partner Warrants issued to the Initial Holder.

     1. CERTAIN DEFINITIONS. Terms defined in the preceding paragraph and
elsewhere in this Warrant have the respective meanings provided for therein. The
following additional terms, as used herein, have the following respective
meanings:

     "Act" means the Securities Act of 1933, as amended.

     "Closing Price" means the closing price per share of the Common Stock on
the principal national securities exchange on which the Common Stock is listed
or admitted to trading or, if not listed or traded on any such exchange, on the
Nasdaq National Market, or if not
<PAGE>

listed or traded on any such exchange or system, the average of the bid and
asked price per share on Nasdaq National Market or, if such quotations are not
available, the fair market value as reasonably determined by the Board of
Directors of the Company or any committee of such Board.

     "Common Stock" means the fully paid and nonassessable shares of common
stock of the Company, par value $.01 per share, together with any other equity
securities that may be issued by the Company in addition thereto or in
substitution therefor, as provided herein.

     "Exchange Act" means the Securities Exchange Act of 1934, as amended.

     "Exercise Period" means the period beginning on April 1, 1994 and ending on
March 31, 2001

     "Exercise Price" means $2.50 per share for 1,000 shares and $3.50 per share
for 1,900 shares until the Exercise Price Escalation Date at which time the
Exercise Price shall mean $8.00, all subject to change or adjustment pursuant to
Section 8 hereof.

     "Exercise Price Escalation Date" means the date 90 days after the date on
which the Company notifies the Holder that the Closing Price has been greater
than or equal to $12.00 for any twenty (20) out of thirty (30) consecutive
trading days.

     "Nasdaq" means the National Association of Securities Dealers Automated
Quotation System.

     "National Market" means the National Market of Nasdaq.

     "Reorganization Event" means (i) any capital reorganization or leveraged
recapitalization of the Company or reclassification of the Common Stock (other
than a subdivision, combination or reclassification of the outstanding Common
Stock for which adjustment is provided in Section 8(a) hereof and other than a
change in the par value of the Common Stock or an increase in the authorized
capital stock of the Company not involving the issuance of any shares thereof),
(ii) any consolidation of the Company with, or merger of the Company with or
into, another person (including any individual, partnership, joint venture,
corporation, trust or group thereof) (other than a consolidation or merger with
a subsidiary of the Company in which the Company is the continuing corporation
for which adjustment is provided in Section 8(a) hereof) or any sale, lease,
transfer or conveyance of all or substantially all of the property and assets of
the Company or (iii) the announcement or commencement by any "person" or "group"
(within the meaning of Section 13(d) and Section 14(d) of the Exchange Act) of a
bona fide tender offer or exchange offer in accordance with the rules and
regulations of the Exchange Act to purchase, or the acquisition of securities in
the Company, such that after such acquisition or proposed purchase, the acquiror
"beneficially owns" or would "beneficially own" (as defined in Rule 13d-3 under
the Exchange Act), securities in the Company representing 30% or more of the
combined voting power of the Company's then outstanding securities having power


                                      -2-
<PAGE>

to vote in the election of directors.

     "Warrant" means one of the warrants issued by the Company pursuant to the
Modification Offer to replace the Exchange Warrants which replaced the Existing
Warrants issued in connection with the initial sale of limited partnership
interests in the Partnership. The term "Warrant" includes this Warrant and any
Warrant or Warrants which may be issued pursuant to Section 5 hereof in
substitution or exchange for or upon transfer of this Warrant, any Warrant which
may be issued pursuant to Section 2 hereof upon partial exercise of this Warrant
and any Warrant which may be issued pursuant to Section 6 hereof upon the loss,
theft, destruction or mutilation of this Warrant.

     "Warrant Register" means the register maintained at the principal office of
the Company, or at the office of its agent, in which the name of the Holder of
this Warrant shall be registered.

     "Warrant Shares" means the shares of Common Stock, as adjusted from time to
time, deliverable upon exercise of this Warrant.

     2. EXERCISE OF WARRANT. This Warrant may be exercised, in whole or in part,
at any time or from time to time during the Exercise Period, by presentation and
surrender hereof to the Company at its principal office at the address set forth
on the signature page hereof (or at such other address of the Company or any
agent appointed by the Company to act hereunder as the Company or such agent may
hereafter designate in writing to the Holder), with the purchase form annexed
hereto (the "Purchase Form") duly executed and accompanied by cash or a
certified or official bank check drawn to the order of "REPLIGEN CORPORATION"
(or its successor in interest, if any) in the amount of the Exercise Price,
multiplied by the number of Warrant Shares specified in such Purchase Form. If
this Warrant should be exercised in part only, the Company or its agent shall,
upon surrender of this Warrant, execute and deliver a Warrant evidencing the
right of the Holder thereof to purchase the balance of the Warrant Shares
purchasable hereunder. Upon receipt by the Company during the Exercise Period of
this Warrant and such Purchase Form in proper form for exercise, together with
proper payment of the Exercise Price at its principal office, or by its agent at
its office, the Holder shall be deemed to be the holder of record of the number
of Warrant Shares specified in such Purchase Form; PROVIDED, HOWEVER, that if
the date of such receipt by the Company or its agent is a date on which the
stock transfer books of the Company are closed, such person shall be deemed to
have become the record holder of such shares on the next business day on which
the stock transfer books of the Company are open. The Company shall pay any and
all documentary, stamp or similar issue or transfer taxes payable in respect of
the issue or delivery of such Warrant Shares. Any Warrant issued upon partial
exercise of this Warrant pursuant to this Section 2 shall be dated the date of
this Warrant.

     3. RESERVATION OF SHARES. The Company agrees that at all times it will keep
reserved solely for issuance and delivery pursuant to the Warrants the number of
shares of its Common Stock that are or would be issuable from time to time upon
exercise of all Warrants


                                      -3-

<PAGE>

issued in connection with the initial sale of limited partnership interests in
the Partnership. All such shares shall be duly authorized and, when issued upon
such exercise, shall be validly issued, fully paid and nonassessable, free and
clear of all liens, security interests, charges and other encumbrances or
restrictions on sale and free of all preemptive rights. Before taking any action
that would cause an adjustment pursuant to Section 8 hereof reducing the
Exercise Price below the then par value (if any) of the Warrant Shares issuable
upon exercise of this Warrant, the Company will take any corporate action that
may, in the opinion of its counsel, be necessary in order that the Company may
validly and legally issue fully paid and nonassessable Warrant Shares at the
Exercise Price as so adjusted.

     4. WARRANT SHARE REGISTRATION RIGHTS.

     (a) The Company has (i) prepared and filed with the Securities and Exchange
Commission (the "SEC") under the Act a registration statement with respect to
the Warrant Shares issuable upon exercise of all Warrants issued in connection
with the initial sale of limited partnership interests in the Partnership and
all Warrants issued to replace such Existing Warrants and will use its best
efforts to cause such registration statement to remain effective under the Act
during the Exercise Period while any of such Warrants are outstanding and (ii)
register or qualify such Warrant Shares under the securities or Blue Sky laws of
each jurisdiction within the United States in which such registration or
qualification is necessary in connection with the issuance and delivery of such
Warrant Shares to the Holders of the Warrants.

     (b) In connection with the registration and qualification referred to in
subsection (a) of this Section 4, the Company covenants and agrees:

          (i) as expeditiously as possible, to prepare and file with the SEC
     such amendments and supplements to such registration statement and the
     prospectus used in connection therewith as may be necessary to keep such
     registration statement effective throughout the Exercise Period;

          (ii) as expeditiously as possible, to take such action as may be
     necessary or desirable to maintain the registration and qualification of
     the Warrant Shares under the securities or Blue Sky laws of the
     jurisdictions referred to in subsection (a) of this Section 4; and

          (iii) to pay all expenses incurred by the Company in complying with
     subsection (a) of this Section 4 and this subsection (b), including (A) all
     registration and filing fees, (B) all printing expenses, (C) all fees and
     disbursements of its counsel and independent public accountants and (D) all
     Blue Sky fees and expenses (including fees and disbursements of counsel).

     5. EXCHANGE, TRANSFER OR ASSIGNMENT OF WARRANT.

     (a) If the Holder has received an opinion of counsel satisfactory to the


                                      -4-

<PAGE>

Company that this Warrant may be freely sold or transferred without registration
under the Act, this Warrant may be, at the option of the Holder, and upon
presentation and surrender hereof to the Company or at the office of its stock
transfer agent, (i) exchanged for other Warrants of different denominations,
entitling the Holder or Holders thereof to purchase in the aggregate the same
number of Warrant Shares at the Exercise Price or (ii) if delivered together
with a written notice specifying the denominations in which new Warrants are to
be issued and signed by the Holder hereof, divided or combined with other
Warrants that carry the same rights.

     (b) If the Holder has received an opinion of counsel satisfactory to the
Company that this Warrant may be freely sold or transferred without registration
under the Act, this Warrant may be assigned, at the option of Holder, upon
surrender of this Warrant to the Company or at the office of its stock transfer
agent, with the Warrant Assignment Form annexed hereto duly executed and
accompanied by funds sufficient to pay any transfer tax. The Company shall
execute and deliver a new Warrant or Warrants in the name of the assignee or
assignees named in such instrument of assignment and, if the Holders's entire
Interest is not being transferred or assigned, in the name of the Holder, and
this Warrant shall promptly be canceled.

     (c) Any transfer or exchange of this Warrant shall be without charge to the
Holder and any new Warrant or Warrants issued pursuant to this Section 5 shall
be dated the date hereof.

     6. LOST, MUTILATED OR MISSING WARRANT. Upon receipt by the Company or its
agent of evidence satisfactory to it of the loss, theft or destruction of this
Warrant, and of satisfactory indemnification, and upon surrender and
cancellation of this Warrant if mutilated, the Company or its agent shall
execute and deliver a Warrant of like tenor and date in exchange for this
Warrant.

     7. RIGHTS OF THE HOLDER. The Holder shall not, by virtue hereof, be
entitled to any rights of a shareholder in the Company, either at law or in
equity, and the rights of the Holder are limited to those expressed in this
Warrant.

     8. ANTI-DILUTION. (a) If the Company shall fix or have fixed a record date
at any time after the date hereof and before the expiration of the Exercise
Period for:

          (i) STOCK DIVIDENDS, SUBDIVISIONS, COMBINATIONS, RECLASSIFICATIONS,
     ETC. (A) The declaration of a dividend or distribution on the Common Stock
     (or other securities deliverable hereunder) payable in shares of capital
     stock (whether shares of Common Stock or of capital stock of any other
     class), (ii) the subdivision of shares of the Common Stock into a greater
     number of shares, (iii) the combination of the Common Stock into a smaller
     number of shares or (iv) the issuance of any shares of its capital stock by
     reclassification of the Common Stock in connection with a consolidation or
     merger with a subsidiary of the Company in which the Company is the
     continuing corporation, then, in any such event, the Holder shall be
     entitled to receive the aggregate number and kind of shares which, if the
     Warrant had been exercised immediately prior to such record


                                      -5-

<PAGE>

     date, he would have been entitled to receive by virtue of such dividend,
     distribution, subdivision, combination or reclassification, and the
     Exercise Price shall be appropriately adjusted. Such adjustment shall be
     made successively whenever any event listed above shall occur.

          (ii) ISSUANCE AT LESS THAN CURRENT MARKET PRICE. The issuance of
     Common Stock (or other securities deliverable hereunder), rights, options
     (excluding options issued in connection with an employee stock option or
     similar plan) or warrants to all holders of Common Stock (or such other
     securities deliverable hereunder) entitling them to subscribe for or
     purchase Common Stock (or such other securities) at a price per share or
     having a conversion price per share less than the Closing Price on such
     record date (excluding rights or warrants that are not immediately
     exercisable and for which provision is made for the Holder to receive
     comparable rights or warrants), then the number of Warrant Shares to be
     received hereunder after such record date shall be determined by
     multiplying the number of shares receivable hereunder immediately prior to
     such record date by a fraction, the denominator of which shall be the
     number of shares of Common Stock (or such other securities deliverable
     hereunder) outstanding on such record date plus the number of shares of
     Common Stock (or such other securities) that the aggregate offering price
     of the total number of shares so offered for subscription or purchase would
     purchase at such Closing Price, and the numerator of which shall be the
     number of shares of Common Stock (or such other securities) outstanding on
     such record date plus the number of additional shares of Common Stock (or
     such other securities) offered for subscription or purchase, and the
     Exercise Price shall be appropriately adjusted. Shares of Common Stock
     owned by or held for the account of the Company or any subsidiary of the
     Company on such record date shall not be deemed outstanding for the purpose
     of any such computation. Such adjustment shall become effective immediately
     after such record date. Such adjustment shall be made successively whenever
     any such event shall occur. If such rights or warrants are not so issued,
     the number of Warrant Shares receivable hereunder shall again be adjusted
     to be the number that would have been in effect had such record date not
     been fixed. On the expiration of such rights or warrants the number of
     Warrant Shares receivable hereunder shall be adjusted to be the number that
     would have obtained had the adjustment made upon the issuance of such
     rights or warrants been made upon the basis of the issuance of only the
     number of shares of Common Stock (or such other securities deliverable
     hereunder) actually issued upon the exercise of such rights or warrants. In
     any case in which this subsection (ii) shall require that an adjustment in
     the number of shares receivable hereunder or the Exercise Price be made
     effective as of a record date for a specified event, the Company may elect
     to defer until the occurrence of such event issuing to the Holder of any
     Warrant exercised after such record date the number of Warrant Shares, if
     any, issuable upon such exercise over and above the number of Warrant
     Shares, if any, issuable upon such exercise on the basis of the Exercise
     Price in effect prior to such adjustment; PROVIDED, HOWEVER, that the
     Company shall deliver to such Holder a due bill or other appropriate
     instrument evidencing such Holder's right to receive such additional
     Warrant Shares upon the occurrence of the event requiring such adjustments.


                                      -6-

<PAGE>

          (iii) DISTRIBUTION OF SUBSCRIPTION RIGHTS, WARRANTS, EVIDENCES OF
     INDEBTEDNESS OR ASSETS. The making of a distribution to all holders of
     Common Stock (or other securities deliverable hereunder) (including any
     such distribution to be made in connection with a consolidation or merger
     in which the Company is to be the continuing corporation) of (A) any shares
     of capital stock of the Company (other than Common Stock), (B) subscription
     rights or warrants (excluding those for which adjustment is provided in
     subsection 8(a)(ii) above and excluding those that are not immediately
     exercisable and for which provision is made for the Holder to receive
     comparable subscription rights or warrants) or (C) evidences of its
     indebtedness or assets (excluding (x) dividends paid in or distributions of
     the Company's capital stock for which the number of Warrant Shares
     receivable hereunder shall have been adjusted pursuant to paragraph 8(a)(i)
     and (y) cash dividends or distributions payable out of earnings or surplus
     not in excess of 10% of the average Closing Price for the thirty trading
     days prior to the fifth day before the date of declaration) any of the
     foregoing being hereinafter in this paragraph (iii) called the
     "Securities"), then in each such case (unless the Company elects to reserve
     shares or other units of such Securities for distribution to each Holder
     upon exercise of the Warrant so that, in addition to the shares of the
     Common Stock (or other securities deliverable hereunder) to which each
     Holder is entitled, each Holder will receive upon such exercise the amount
     and kind of such Securities which such Holder would have received if the
     Company had, immediately prior to the record date for the distribution of
     the Securities, exercised the Warrant) the number of Warrant Shares
     receivable hereunder after such record date shall be determined by
     multiplying the number of Warrant Shares receivable hereunder immediately
     prior to such record date by a fraction, the denominator of which shall be
     the Closing Price on the trading day immediately prior to the date the
     Common Stock (or such other securities deliverable hereunder) trades
     without the right to receive such Securities, less the fair market value
     (as determined in the reasonable judgment of the Board of Directors of the
     Company and described in a statement mailed by certified mail to the
     Holder) of the portion of the assets or evidences of indebtedness so to be
     distributed to a holder of one share of the Common Stock or of such
     subscription rights or warrants applicable to one share of the Common
     Stock, and the numerator of which shall be the Closing Price of the Common
     Stock on such trading date; and the Exercise Price shall be appropriately
     adjusted. Such adjustment shall become effective immediately after such
     record date and shall be made successively whenever such a record date is
     fixed. If such distribution is not so made, the number of Warrant Shares
     receivable hereunder shall again be adjusted to be the number that was in
     effect immediately prior to such record date. In any case in which this
     paragraph (iii) shall require that an adjustment in the number of Warrant
     Shares receivable hereunder or the Exercise Price be made effective as of a
     record date for a specified event, the Company may elect to defer until the
     occurrence of such event issuing to the Holder of any Warrant exercised
     after such record date the number of Warrant Shares, if any, issuable upon
     such exercise over and above the number of Warrant Shares, if any, issuable
     upon such exercise on the basis of the Exercise Price in effect prior to
     such adjustment; PROVIDED, HOWEVER, that the Company shall deliver to


                                      -7-
<PAGE>

     such Holder a due bill or other appropriate instrument evidencing such
     Holder's right to receive such additional Warrant Shares upon the
     occurrence of the event requiring such adjustments.

     (b) REORGANIZATION EVENT.

          (i) In case of any Reorganization Event the Company shall, as a
     condition precedent to the consummation of the transaction constituting, or
     announced as, such Reorganization Event, cause effective provisions to be
     made so that the Holder shall have the right immediately thereafter, by
     exercising this Warrant, to receive the aggregate amount and kind of shares
     of stock and other securities and property that were receivable upon such
     Reorganization Event by a holder of the number of shares' of Common Stock
     that would have been received immediately prior to such Reorganization
     Event upon exercise of this Warrant. Any such provision shall include
     provision for adjustments in respect of such shares of stock and other
     securities and property that shall be as nearly equivalent as may be
     practicable to the adjustments provided for in Section 8(a). The foregoing
     provisions of this Section 8(b) shall similarly apply to successive
     Reorganization Events.

          (ii) The Company shall, at least twenty days before earliest of (1)
     the date on which a Reorganization Event occurs or (2) the date on which
     the Company shall agree to effect a Reorganization Event (PROVIDED that if
     approval of the shareholders of the Company is required in connection with
     such Reorganization Event then on the date of such approval and PROVIDED
     FURTHER that if such Reorganization Event was beyond the control of the
     Company, and the Company did not have knowledge twenty days before the date
     of such Reorganization Event, as soon as practicable thereafter), cause to
     be mailed to the Holder a notice describing in reasonable detail such
     Reorganization Event and informing the Holder of his or her rights pursuant
     to Section 8(b)(i) above.

     (c) FRACTIONAL SHARES. No fractional shares of Common Stock (or other
securities deliverable hereunder) or scrip shall be issued to any Holder in
connection with the exercise of this Warrant. Instead of any fractional share of
Common Stock (or other securities deliverable hereunder) that would otherwise be
issuable to such Holder, the Company shall pay to such Holder a cash adjustment
in respect of such fractional interest in an amount equal to such fractional
interest multiplied by the then current Closing Price per share of Common Stock
(or other securities deliverable hereunder).

     (d) CARRYOVER. Notwithstanding any other provision of this Section 8, no
adjustment shall be made to the number of shares of Common Stock (or other
securities deliverable hereunder) to be delivered to each Holder (or to the
Exercise Price) if such adjustment would represent less than one percent of the
number of shares to be so delivered, but any such adjustment shall be carried
forward and shall be made at the time and together with the next subsequent
adjustment which, together with any adjustments so carried forward, shall amount
to one percent or more of the number of shares to be so delivered.


                                      -8-
<PAGE>

     (e) NOTICES OF CERTAIN EVENTS. If at any time after the date hereof and
before the expiration of the Exercise Period:

          (i) the Company authorizes the issuance to all holders of its Common
     Stock of (A) rights or warrants to subscribe for or purchase shares of its
     Common Stock or (B) any other subscription rights or warrants; or

          (ii) the Company authorizes the distribution to all holders of its
     Common Stock of evidences of its indebtedness or assets (other than cash
     dividends or distributions excluded from the operation of paragraph
     8(a)(iii); or

          (iii) there shall be any capital reorganization of the Company or
     reclassification of the Common Stock (other than a change in par value of
     the Common Stock or an increase in the authorized capital stock of the
     Company not involving the issuance of any shares thereof) or any
     consolidation or merger to which the Company is a party (other than a
     consolidation or merger with a subsidiary in which the Company is the
     continuing corporation and that does not result in any reclassification or
     change in the Common Stock outstanding) or a conveyance or transfer of all
     or substantially all of the properties and assets of the Company;

          (iv) there shall be any voluntary or involuntary dissolution,
     liquidation or winding-up of the Company; or

          (v) there shall be any other event that would result in an adjustment
     pursuant to this Section 8 in the Exercise Price or the number of Warrant
     Shares that may be purchased upon the exercise hereof;

the Company will cause to be mailed to the Holder, at least twenty days (or ten
days in any case specified in clauses (i) or (ii) above) before the applicable
record or effective date hereinafter specified, a notice stating (A) the date as
of which the holders of Common Stock of record entitled to receive any such
rights, warrants or distributions is to be determined, or (B) the date on which
any such reorganization, reclassification, consolidation, merger, conveyance,
transfer, dissolution, liquidation or winding-up is expected to become
effective, and the date as of which it is expected that holders of Common Stock
of record will be entitled to exchange their shares of Common Stock for
securities or other property, if any, deliverable upon such reorganization,
reclassification, consolidation, merger, conveyance, transfer, dissolution,
liquidation or winding-up.

     (f) FAILURE TO GIVE NOTICE. The failure to give the notice required by
Section 8(e) hereof or any defect therein shall not affect the legality or
validity of any distribution right, warrant, consolidation, merger, conveyance,
transfer, dissolution, liquidation or winding-up or the vote upon any such
action.


                                      -9-
<PAGE>

     9. OFFICERS' CERTIFICATE. Whenever the number of Warrant Shares that may be
purchased on exercise of this Warrant or the Exercise Price is adjusted as
required by the provisions of Section 8 hereof, the Company will forthwith file
in the custody of its Secretary or an Assistant Secretary at its principal
office and at the office of its agent an officers' certificate showing the
adjusted number of Warrant Shares that may be purchased at the Exercise Price on
exercise of this Warrant and the adjusted Exercise Price determined as herein
provided, setting forth in reasonable detail the facts requiring such adjustment
and the manner of computing such adjustment. Each such officers' certificate
shall be signed by the President, Chief Financial Officer or Treasurer of the
Company and by the Secretary or an Assistant Secretary of the Company. Each such
officers' certificate shall be made available at all reasonable times for
inspection by the Holder. The Company shall, forthwith after each such
adjustment, cause such certificate to be mailed to the Holder.

     10. LISTING ON SECURITIES EXCHANGES. The Company will list on each national
securities exchange on which any Common Stock may at any time be listed or
quoted and have authorized for quotation on the National Market (if any Common
Stock is then authorized for quotation thereon) all shares of Common Stock from
time to time issuable upon exercise of all Warrants issued in connection with
the initial sale of the limited partnership interests in the Partnership,
subject to official notice of issuance (if required), and will maintain such
listing or quotation so long as any other shares of its Common Stock are so
listed or quoted. The Company shall so list or have authorized for quotation,
and shall maintain such listing or quotation of, any other shares of capital
stock of the Company issuable upon the exercise of this Warrant if and so long
as any shares of capital stock of the same class are so listed or quoted. Any
such listing or quotation shall be at the Company's expense.

     11. AVAILABILITY OF INFORMATION. The Company shall comply with all
applicable public information reporting requirements of the SEC and applicable
state securities laws (including those required to make available the benefits
of Rule 144 under the Act) to which it may from time to time be subject. The
Company will also cooperate with each Holder of any Warrants and each holder of
any Warrant Shares in supplying such information concerning the Company as may
be necessary for such Holder or holder to complete and file any information
reporting forms currently or hereafter required by the SEC as a condition to the
availability of an exemption from the Act for the sale of any Warrants or
Warrant Shares.

     12. WARRANT REGISTER. The Company will register this Warrant in the Warrant
Register in the name of the record holder to whom it has been distributed or
assigned in accordance with the terms hereof. The Company may deem and treat the
registered Holder of this Warrant as the absolute owner hereof (notwithstanding
any notation of ownership or other writing hereon made by anyone) for the
purpose of any exercise hereof or any distribution to the Holder and for all
other purposes, and the Company shall not be affected by any notice to the
contrary.

     13. SUCCESSORS. All of the provisions of this Warrant by or for the benefit
of the Company or the Holder shall bind and


                                      -10-
<PAGE>

inure to the benefit of their respective successors and assigns.

     14. HEADINGS. The headings of sections of this Warrant have been inserted
for convenience of reference only, are not to be considered a part hereof and
shall in no way modify or restrict any of the terms or provisions hereof.

     15. AMENDMENTS. This Warrant may be amended by the affirmative vote of
Holders holding Warrants to purchase not less than two-thirds of the Warrant
Shares purchasable pursuant to all of the then outstanding Warrants; PROVIDED,
that, except as expressly provided herein, this Warrant may not be amended,
without the consent of the Holder, to change (i) any price at which this Warrant
may be exercised, (ii) the period during which this Warrant may be exercised,
(iii) the number or type of securities to be issued upon the exercise hereof or
(iv) the provision of this Section 15.

     16. NOTICES. Unless otherwise provided in this Warrant, any notice or other
communication required or permitted to be made or given to any party hereto
pursuant to this Warrant shall be in writing and shall be deemed made or given
if delivered by hand, on the date of such delivery to such party or, if mailed,
on the fifth day after the date of mailing, if sent to such party by certified
or registered mail, postage prepaid, addressed to it (in the case of a Holder)
at its address in the Warrant Register or (in the case of the Company) at its
address set forth below, or to such other address as is designated by written
notice, similarly given to each other party hereto.

     17. GOVERNING LAW. This Warrant shall be deemed to be a contract made under
the laws of the State of Delaware and for all purposes shall be construed in
accordance with the laws of said State as applied to contracts made and to be
performed in Delaware between Delaware residents.


                                      -11-
<PAGE>

     18. SIGNATURES. This Warrant and any Warrant issued pursuant to the terms
hereof shall be manually signed, or shall bear the facsimile signature of the
President or a Vice President of the Company which shall have the same effect as
if such Warrant were manually signed.

     IN WITNESS WHEREOF, the Company has duly caused this Warrant to be signed
and attested by its duly authorized officers and to be dated as of
_______________.

                                         REPLIGEN CORPORATION

                                         By __________________________________
                                            Title: President and CEO

                                            Address:

                                                Repligen Corporation
                                                117 Fourth Avenue
                                                Needham, MA 02494

                                            Attention:  Secretary


                                      -12-
<PAGE>

                                  PURCHASE FORM

     The undersigned, _________________ hereby irrevocably elects to exercise
the within Warrant to purchase _____ shares of Common Stock and hereby makes
payment of $___________ in payment of the exercise price thereof.

Date:  ____________, 19__                ___________________________________
                                         [Signed]



                                         ___________________________________
                                         [Street Address]



                                         ___________________________________
                                         [City and State]


                                      -13-
<PAGE>

                             WARRANT ASSIGNMENT FORM

     FOR VALUE RECEIVED, the undersigned, _________________________,
("Assignor"), hereby sells, assigns and transfers unto

Name:  ____________________________________ ("Assignee")
       (Please type or print in block letters.)

Address:  _________________________________

          _________________________________

Social Security or Taxpayer I.D. No.:  __________________________

Assignor's right to purchase up to _____ shares of Common Stock represented by
this Warrant and does hereby irrevocably constitute and appoint the Company and
any of its officers, secretary, or assistant secretaries, as attorneys-in-fact
to transfer the same on the books of the Company, with full power of
substitution in the premises.

Date:  _______________, 199_             ___________________________________
                                         [Signed]


                                      -14-

<PAGE>

                 [letterhead of Testa, Hurwitz & Thibeault, LLP]

                                                                January 28, 2000

Repligen Corporation
117 Fourth Avenue
Needham, Massachusetts 02194

     Re:   S-3 REGISTRATION STATEMENT

Ladies and Gentlemen:

     We are counsel to Repligen Corporation, a Delaware corporation (the
"Company"), and have represented the Company in connection with the preparation
and filing of the Company's Form S-3 Registration Statement (the "Registration
Statement"), covering the sale to the public of up to 2,007,050 shares of the
Company's Common Stock, $.01 par value per share (the "Shares"), upon the
exercise of the warrants described in the Registration Statement (the
"Warrants").

     We have reviewed the corporate proceedings taken by the Board of Directors
of the Company with respect to the authorization and issuance of the Shares upon
exercise of the Warrants. We have also examined and relied upon originals or
copies, certified or otherwise authenticated to our satisfaction, of all
corporate records, documents, agreements or other instruments of the Company and
have made all investigations of law and have discussed with the Company's
officers all questions of fact that we have deemed necessary or appropriate.

     Based upon and subject to the foregoing, we are of the opinion that the
Shares, when issued upon the exercise of the Warrants in accordance with the
terms of the Registration Statement, will be legally issued, fully paid and
non-assessable.

     We hereby consent to the filing of this opinion as Exhibit 5 to the
Registration Statement and to the reference to our firm in the Prospectus
contained in the Registration Statement under the caption "Legal Matters."

                                Very truly yours,

                                /s/ Testa, Hurwitz & Thibeault, LLP

                                TESTA, HURWITZ & THIBEAULT, LLP

<PAGE>

Exhibit 23.1

                CONSENT OF INDEPENDENT PUBLIC ACCOUNTANTS



    As independent public accountants, we hereby consent to the incorporation
by reference in this registration statement of our report dated May 14, 1999
included in Repligen Corporation's Form 10-K for the year ended March 31,
1999 and to all references to our Firm included in this registration
statement.



                                            ARTHUR ANDERSEN LLP


Boston, Massachusetts
January 27, 2000


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