ORAVAX INC /DE/
S-1/A, 1996-06-20
BIOLOGICAL PRODUCTS, (NO DIAGNOSTIC SUBSTANCES)
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<PAGE>   1
 
   
     AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON JUNE 20, 1996
    
                                                       REGISTRATION NO. 333-3056
================================================================================
 
                       SECURITIES AND EXCHANGE COMMISSION
                              WASHINGTON, D.C. 20549
                               AMENDMENT NO. 2 TO
    
                                    FORM S-1
            REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933

                            ------------------------
 
                                  ORAVAX, INC.
             (Exact name of registrant as specified in its charter)

                            ------------------------
<TABLE>
<CAPTION>

<S>                              <C>                             <C>
      DELAWARE                              2836                    04-3085209
(State or other jurisdiction of  (Primary Standard Industrial     (IRS Employer
incorporation or organization)   Classification Code Number)     Identification  Number)
</TABLE> 
                38 SIDNEY STREET, CAMBRIDGE, MASSACHUSETTS 02139
                                 (617) 494-1339
         (Address, including zip code, and telephone number, including
            area code, of registrant's principal executive offices)
               
                            ------------------------
 
                             LANCE K. GORDON, PH.D.
                     PRESIDENT AND CHIEF EXECUTIVE OFFICER
                                  ORAVAX, INC.
                                38 SIDNEY STREET
                 CAMBRIDGE, MASSACHUSETTS 02139 (617) 494-1339
               (Name, address, including zip code, and telephone
               number, including area code, of agent for service)
 
                           ------------------------
 
                                   Copies to:
 
JOHN M. WESTCOTT, JR., ESQ.                      BRUCE DALLAS, ESQ
    HALE AND DORR                              DAVIS POLK & WARDWELL
   60 State Street                              450 Lexington Avenue
Boston, Massachusetts 02109                  New York, New York 10017
     (617) 526-6000                               (212) 450-4000
 
                            ------------------------
 
        Approximate date of commencement of proposed sale to the public:
            As soon as practicable after the effective date hereof.
                            ------------------------
 
     If any of the securities being registered on this Form are to be offered on
a delayed or continuous basis pursuant to Rule 415 under the Securities Act of
1933, check the following box.  / /
 
     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 Registration statement number of the earlier effective
registration statement for the same offering. / / ------------
 
     If this form is a post effective amendment filed pursuant to Rule 462(c)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering. / / ------------
 
     If delivery of the prospectus is expected to be made pursuant to Rule 434,
please check the following box: / /
 
     THE REGISTRANT HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE OR
DATES AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL THE REGISTRANT SHALL
FILE A FURTHER AMENDMENT WHICH SPECIFICALLY STATES THAT THIS REGISTRATION
STATEMENT SHALL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH SECTION 8(a) OF
THE SECURITIES ACT OF 1933 OR UNTIL THE REGISTRATION STATEMENT SHALL BECOME
EFFECTIVE ON SUCH DATE AS THE COMMISSION, ACTING PURSUANT TO SECTION 8(a), MAY
DETERMINE.
==============================================================================
<PAGE>   2
<TABLE>
 
                                  ORAVAX, INC.
 
              CROSS REFERENCE SHEET SHOWING LOCATION IN PROSPECTUS
             OF INFORMATION REQUIRED BY ITEMS IN PART I OF FORM S-1
 
<CAPTION>
     REGISTRATION STATEMENT
       ITEM AND CAPTION                                    LOCATION IN PROSPECTUS
     ---------------------                                 ----------------------
 <C> <S>                                              <C>       
  1. Forepart of Registration Statement and Outside
     Front Cover Page of Prospectus.................  Outside Front Cover Page
  
  2. Inside Front and Outside Back Cover Pages of
     Prospectus.....................................  Inside Front Cover Page; Outside Back
                                                      Cover Page
  3. Summary Information, Risk Factors and Ratio of
     Earnings to Fixed Charges......................  Prospectus Summary; Risk Factors
  
  4. Use of Proceeds................................  Prospectus Summary; Use of Proceeds
  
  5. Determination of Offering Price................  Underwriting
  
  6. Dilution.......................................  Dilution
  
  7. Selling Security Holders.......................  Principal and Selling Stockholders
  
  8. Plan of Distribution...........................  Outside Front Cover Page; Underwriting
  
  9. Description of Securities to be Registered.....  Description of Capital Stock
 
 10. Interests of Named Experts and Counsel.........  Legal Matters
 
 11. Information With Respect to the Registrant:
 
     (a)   Description of Business..................  Prospectus Summary; Recent Developments;
                                                      Business

     (b)   Description of Property..................  Business -- Facilities
 
     (c)   Legal Proceedings........................  Business -- Legal Proceedings
 
     (d)   Market Price of and Dividends on the
           Registrant's Common Equity and Related
           Stockholder Matters......................  Front Cover Page; Price Range of Common
                                                      Stock; Dividend Policy; Description of
                                                      Capital Stock; Shares Eligible for Future
                                                      Sale; Management -- Executive
                                                      Compensation
     
     (e)   Financial Statements.....................  Financial Statements; Capitalization
 
     (f)   Selected Financial Data..................  Selected Financial Data
 
     (g)   Supplementary Financial Information......  Not Applicable
 
     (h)   Management's Discussion and Analysis of
           Financial Condition and Results of
           Operations...............................  Management's Discussion and Analysis of
                                                      Financial Condition and Results of
                                                      Operations
     (i)   Changes in and Disagreements with
           Accountants on Accounting and Financial
           Disclosure...............................  Not Applicable
 
     (j)   Directors, Executive Officers, Promoters
           and Control Persons......................  Management -- Directors and Executive
                                                      Officers; Certain Transactions

     (k)   Executive Compensation...................  Management -- Executive Compensation
 
     (l)   Security Ownership of Certain Beneficial
           Owners and Management....................  Principal and Selling Stockholders
 
     (m)   Certain Relationships and Related
           Transactions.............................  Certain Transactions
 
 12. Disclosure of Commission Position on
     Indemnification for Securities Act               
     Liabilities....................................  Not Applicable
</TABLE>
<PAGE>   3
 
   
                                EXPLANATORY NOTE
    
 
   
     This Amendment No. 2 to Registration Statement on Form S-1 (File No.
333-3056) of OraVax, Inc. is filed solely to file copies of the exhibits listed
in Item 16 hereto.
    
<PAGE>   4

<TABLE>
ITEM 16.  EXHIBITS AND FINANCIAL STATEMENT SCHEDULES
        
          (a)  EXHIBITS
 
   
<CAPTION>
  EXHIBIT
    NO.                                        DESCRIPTION
  -------                                      -----------
<S>         <C>
 1          Form of Underwriting Agreement.

 3.1*       Restated Certificate of Incorporation of the Registrant, as amended.

 3.2*       Form of Second Amended and Restated Certificate of Incorporation of the
            Registrant.

 3.3*       Amended and Restated By-Laws of the Registrant.

 4*         Specimen Certificate for shares of Common Stock, $.001 par value, of the
            Registrant.

 5++        Opinion of Hale and Dorr with respect to the validity of the securities being
            offered.

10.1*       1990 Stock Option Plan, as amended.

10.2*       1995 Stock Option Plan.

10.3*       1995 Employee Stock Purchase Plan.

10.4*       401(k) Savings and Retirement Plan.

10.5*       Employment Agreement between the Company and Lance K. Gordon dated as of July 19,
            1990.

10.6*       Employment Agreement between the Company and Thomas P. Monath dated October 18,
            1991.

10.7*       Employment Agreement between the Company and Robert B. Rombauer dated May 1,
            1992.

10.8*       Employment Agreement between the Company and Keith S. Ehrlich dated October 8,
            1993.

10.9*       Employment Agreement between the Company and Samuel K. Ackerman dated December 1,
            1993.

10.10*      Stock Purchase Agreement between the Company and Lance K. Gordon dated as of
            September 28, 1990.

10.11***    Letter Agreement dated February 5, 1996 between the Company and Dr. Allen Misher.

10.12*      Master Lease Agreement between Comdisco and the Company dated March 31, 1993,
            together with Series D Stock Warrant.

10.13*      Equipment Schedule to Master Lease Agreement between Comdisco and the Company
            dated June 21, 1994, together with Series E Stock Warrant.

10.14+*     Invention and License Administration Agreement between the President and Fellows
            of Harvard College and the Swiss Institute for Experimental Cancer Research dated
            October 22, 1990.

10.15+*     Research and Development Agreement among the Company, Foundation Pour La
            Recherche Des Maladies Gastro-Intestinales, Dr. Andre Blum and Kieta Holding,
            S.A. dated April 2, 1992, as amended in February 1993 and January 1994.

10.16+*     Invention and License Administration Agreement among Foundation Pour La Recherche
            Des Maladies Gastro-Intestinales, the Swiss Institute for Experimental Cancer
            Research and Dr. Jean-Pierre Kraehenbuhl dated April 2, 1992.

10.17+*     Invention and License Administration Agreement among Foundation Pour La Recherche
            Des Maladies Gastro-Intestinales, Max-Planck Institute for Biology and Dr. Rainer
            Haas dated April 1992 and related Participation Agreement.

10.18*      Participation Agreements by and between the Company and each of Dr. Rainer Haas,
            Sven Gustafson, Dr. Jean-Pierre Kraehenbuhl, Dr. Madeleine Heitz, Dr. Pierre
            Michetti and Dr. Andre Blum.

10.19+*     Agreement between the Company, the Natural Environmental Research Council and Dr.
            D.H.L. Bishop dated May 15, 1992.

10.20+*     Collaborative Development and License Agreement between the Company and Techlab,
            Inc. dated March 31, 1993.

10.21+*     Industrial Research Agreement between the Company, Case Western Reserve
            University, Dr. Steven J. Czinn and Dr. John G. Nedrud dated October 14, 1993.
</TABLE>
    
 
                                      II-1
<PAGE>   5
 
   
<TABLE>
<CAPTION>
  EXHIBIT
    NO.                                        DESCRIPTION
  -------                                      -----------
<S>         <C>
10.22+*     Assignment Agreement among the Company, Dr. Steven J. Czinn and Dr. John G.
            Nedrud dated October 14, 1993.

10.23+*     Cooperative Research and Development Agreement between the Company and Walter
            Reed Army Institute of Research dated December 19, 1994.

10.24+*     Master Agreement between the Company and Pasteur Merieux Serums & Vaccins S.A.
            dated March 31, 1995.

10.25+*     Research and Development Agreement between the Company and Merieux OraVax S.N.C.
            and OraVax, Merieux Co. dated March 31, 1995.

10.26**     Lease dated October 27, 1995 between the Registrant and Forest City 38 Sidney
            Street, Inc.

10.27***    Assignment of Lease dated as of January 1, 1996 between Immunogen, Inc. and the
            Company.

10.28***    Landlord's Waiver, Consent and Agreement dated as of January 1, 1996 by and among
            AEW No. 1 Corporation, Aberlyn Capital Management Limited Partnership and the
            Company.

10.29***    Consent to Assignment and Leasehold Mortgage dated as of January 1, 1996 by and
            among AEW No. 1 Corporation, Immunogen, Inc. and the Company.

10.30++***  License Agreement dated as of February 6, 1996 between Institut Pasteur and the
            Company.

11++        Calculation of shares used in determining historical net loss per common share.

21***       Subsidiaries of the Registrant.

23.1++      Consent of Hale and Dorr (included in Exhibit 5).

23.2++      Consent of Coopers & Lybrand L.L.P.

23.3++      Consent of Fish & Richardson.

24++        Powers of Attorney.
    
<FN> 
- ------------------------------
 
*    Incorporated by reference to the Company's Registration Statement on Form
     S-1, File No. 33-90936.
 
**   Incorporated by reference from Exhibit 10.1 to the Registrant's Quarterly
     Report on Form 10-Q for the fiscal quarter dated September 30, 1995.
 
***  Incorporated by reference to the Company's Annual Report on Form 10-K for
     the fiscal year ended December 31, 1995.
 
   
++    Previously filed.
    
 
+    Confidential treatment granted as to certain portions, which portions are
     omitted and filed separately with the Commission.
 
++   Confidential treatment requested as to certain portions, which portions are
     omitted and filed separately with the Commission.

</TABLE>
 
     (b) FINANCIAL STATEMENT SCHEDULES
 
   
     All schedules have been omitted because they are not required or because
    
the required information is given in the Financial Statements or Notes thereto.
 
                                      II-2
<PAGE>   6
 
                                   SIGNATURES
 
   
     Pursuant to the requirements of the Securities Act of 1933, as amended, the
Registrant has duly caused this Amendment No. 2 to Registration Statement to be
signed on its behalf by the undersigned, thereunto duly authorized, in the City
of Cambridge, Commonwealth of Massachusetts, on this 20th day of June, 1996.
    
 
                                          ORAVAX, INC.
 
                                          By: /s/  LANCE K. GORDON
                                            ------------------------------------
                                               Lance K. Gordon
                                               President and Chief
                                               Executive Officer

<TABLE>
     Pursuant to the requirements of the Securities Act of 1933, as amended,
this Amendment No. 2 to Registration Statement has been signed below by the
following persons in the capacities and on the dates indicated.
 
   
<CAPTION>
                SIGNATURE                             TITLE                        DATE
<S>                                           <C>                               <C>

/s/  LANCE K. GORDON                          President, Chief Executive        June 20, 1996
- ------------------------------------------    Officer and Director
Lance K. Gordon                               (Principal Executive Officer)

/s/  KEITH S. EHRLICH                         Vice President, Finance and       June 20, 1996
- ------------------------------------------    Administration, Treasurer and
Keith S. Ehrlich                              Chief Financial Officer
                                              (Principal Financial and
                                              Accounting Officer)

DOUGLAS MACMASTER*                            Director                          June 20, 1996
- ------------------------------------------
Douglas MacMaster

RICHARD A. CHARPIE*                           Director                          June 20, 1996
- ------------------------------------------
Richard A. Charpie

JEAN J. DELEAGE*                              Director                          June 20, 1996
- ------------------------------------------
Jean J. Deleage

ANDRE L. LAMOTTE*                             Director                          June 20, 1996
- ------------------------------------------
Andre L. Lamotte

ALLEN MISHER*                                 Director                          June 20, 1996
- ------------------------------------------
Allen Misher
</TABLE>
    
 
*By: /s/  LANCE K. GORDON
     ------------------------------
          Lance K. Gordon
        Attorney-in-fact
 
                                      II-3
<PAGE>   7

   
                                 EXHIBIT INDEX
<TABLE>
<CAPTION>

  EXHIBIT
    NO.                                        DESCRIPTION
  -------                                      -----------
<S>         <C>
 1          Form of Underwriting Agreement.

 3.1*       Restated Certificate of Incorporation of the Registrant, as amended.

 3.2*       Form of Second Amended and Restated Certificate of Incorporation of the
            Registrant.

 3.3*       Amended and Restated By-Laws of the Registrant.

 4*         Specimen Certificate for shares of Common Stock, $.001 par value, of the
            Registrant.

 5++        Opinion of Hale and Dorr with respect to the validity of the securities being
            offered.

10.1*       1990 Stock Option Plan, as amended.

10.2*       1995 Stock Option Plan.

10.3*       1995 Employee Stock Purchase Plan.

10.4*       401(k) Savings and Retirement Plan.

10.5*       Employment Agreement between the Company and Lance K. Gordon dated as of July 19,
            1990.

10.6*       Employment Agreement between the Company and Thomas P. Monath dated October 18,
            1991.

10.7*       Employment Agreement between the Company and Robert B. Rombauer dated May 1,
            1992.

10.8*       Employment Agreement between the Company and Keith S. Ehrlich dated October 8,
            1993.

10.9*       Employment Agreement between the Company and Samuel K. Ackerman dated December 1,
            1993.

10.10*      Stock Purchase Agreement between the Company and Lance K. Gordon dated as of
            September 28, 1990.

10.11***    Letter Agreement dated February 5, 1996 between the Company and Dr. Allen Misher.

10.12*      Master Lease Agreement between Comdisco and the Company dated March 31, 1993,
            together with Series D Stock Warrant.

10.13*      Equipment Schedule to Master Lease Agreement between Comdisco and the Company
            dated June 21, 1994, together with Series E Stock Warrant.

10.14+*     Invention and License Administration Agreement between the President and Fellows
            of Harvard College and the Swiss Institute for Experimental Cancer Research dated
            October 22, 1990.

10.15+*     Research and Development Agreement among the Company, Foundation Pour La
            Recherche Des Maladies Gastro-Intestinales, Dr. Andre Blum and Kieta Holding,
            S.A. dated April 2, 1992, as amended in February 1993 and January 1994.

10.16+*     Invention and License Administration Agreement among Foundation Pour La Recherche
            Des Maladies Gastro-Intestinales, the Swiss Institute for Experimental Cancer
            Research and Dr. Jean-Pierre Kraehenbuhl dated April 2, 1992.

10.17+*     Invention and License Administration Agreement among Foundation Pour La Recherche
            Des Maladies Gastro-Intestinales, Max-Planck Institute for Biology and Dr. Rainer
            Haas dated April 1992 and related Participation Agreement.

10.18*      Participation Agreements by and between the Company and each of Dr. Rainer Haas,
            Sven Gustafson, Dr. Jean-Pierre Kraehenbuhl, Dr. Madeleine Heitz, Dr. Pierre
            Michetti and Dr. Andre Blum.

10.19+*     Agreement between the Company, the Natural Environmental Research Council and Dr.
            D.H.L. Bishop dated May 15, 1992.

10.20+*     Collaborative Development and License Agreement between the Company and Techlab,
            Inc. dated March 31, 1993.

10.21+*     Industrial Research Agreement between the Company, Case Western Reserve
            University, Dr. Steven J. Czinn and Dr. John G. Nedrud dated October 14, 1993.
</TABLE>
    
 
<PAGE>   8
 
   
<TABLE>
<CAPTION>
  EXHIBIT
    NO.                                        DESCRIPTION
  -------                                      -----------
<S>         <C>
10.22+*     Assignment Agreement among the Company, Dr. Steven J. Czinn and Dr. John G.
            Nedrud dated October 14, 1993.

10.23+*     Cooperative Research and Development Agreement between the Company and Walter
            Reed Army Institute of Research dated December 19, 1994.

10.24+*     Master Agreement between the Company and Pasteur Merieux Serums & Vaccins S.A.
            dated March 31, 1995.

10.25+*     Research and Development Agreement between the Company and Merieux OraVax S.N.C.
            and OraVax, Merieux Co. dated March 31, 1995.

10.26**     Lease dated October 27, 1995 between the Registrant and Forest City 38 Sidney
            Street, Inc.

10.27***    Assignment of Lease dated as of January 1, 1996 between Immunogen, Inc. and the
            Company.

10.28***    Landlord's Waiver, Consent and Agreement dated as of January 1, 1996 by and among
            AEW No. 1 Corporation, Aberlyn Capital Management Limited Partnership and the
            Company.

10.29***    Consent to Assignment and Leasehold Mortgage dated as of January 1, 1996 by and
            among AEW No. 1 Corporation, Immunogen, Inc. and the Company.

10.30++***  License Agreement dated as of February 6, 1996 between Institut Pasteur and the
            Company.

11++        Calculation of shares used in determining historical net loss per common share.

21***       Subsidiaries of the Registrant.

23.1++      Consent of Hale and Dorr (included in Exhibit 5).

23.2++      Consent of Coopers & Lybrand L.L.P.

23.3++      Consent of Fish & Richardson.

24++        Powers of Attorney.
    
<FN> 
- ------------------------------
 
*    Incorporated by reference to the Company's Registration Statement on Form
     S-1, File No. 33-90936.
 
**   Incorporated by reference from Exhibit 10.1 to the Registrant's Quarterly
     Report on Form 10-Q for the fiscal quarter dated September 30, 1995.
 
***  Incorporated by reference to the Company's Annual Report on Form 10-K for
     the fiscal year ended December 31, 1995.
 
   
++    Previously filed.
    
 
+    Confidential treatment granted as to certain portions, which portions are
     omitted and filed separately with the Commission.
 
++   Confidential treatment requested as to certain portions, which portions are
     omitted and filed separately with the Commission.

</TABLE>
 

<PAGE>   1
                               [2,544,700] Shares

                                  ORAVAX, INC.

                                  Common Stock

                             UNDERWRITING AGREEMENT


                                                                 June  , 1996

COWEN & COMPANY
DONALDSON, LUFKIN & JENRETTE SECURITIES CORPORATION
     As Representatives of the several Underwriters

c/o Cowen & Company
     Financial Square
     New York, New York 10005

Dear Sirs:

          1. INTRODUCTORY. Oravax, Inc., a Delaware corporation (the "Company"),
and the selling stockholders named in Schedule B hereto (the "Selling
Stockholders") propose to sell, pursuant to the terms of this Agreement, to the
several underwriters named in Schedule A hereto (the "Underwriters," or, each,
an "Underwriter"), an aggregate of [2,544,700] shares of Common Stock, $.001 par
value (the "Common Stock") of the Company. The aggregate of [2,544,700] shares
so proposed to be sold is hereinafter referred to as the "Firm Stock". The
Company also proposes to sell to the Underwriters, upon the terms and conditions
set forth in Section 3 hereof, up to an additional [381,705] shares of Common
Stock (the "Optional Stock"). The Firm Stock and the Optional Stock are
hereinafter collectively referred to as the "Stock". Cowen & Company ("Cowen")
and Donaldson, Lufkin & Jenrette Securities Corporation are acting as
representatives of the several Underwriters and in such capacity are hereinafter
referred to as the "Representatives".

          2. (a) REPRESENTATIONS AND WARRANTIES OF THE COMPANY. The Company
represents and warrants to, and agrees with, the several Underwriters that:

               (i) A registration statement on Form S-1 (File No. 333-3056) in
          the form in which it became or becomes effective and also in such form
          as it may be when any post-effective amendment thereto shall become
          effective with respect to the Stock, including any preeffective
          prospectuses included as part of the registration statement as




<PAGE>   2



          originally filed or as part of any amendment or supplement thereto, or
          filed pursuant to Rule 424 under the Securities Act of 1933, as
          amended (the "Securities Act"), and the rules and regulations (the
          "Rules and Regulations") of the Securities and Exchange Commission
          (the "Commission") thereunder, copies of which have heretofore been
          delivered to you, has been prepared by the Company in conformity with
          the requirements of the Securities Act and has been filed with the
          Commission under the Securities Act. If it is contemplated, at the
          time this Agreement is executed, that a post-effective amendment to
          the registration statement will be filed and must be declared
          effective before the offering of the Stock may commence, the term
          "Registration Statement" as used in this Agreement means the
          registration statement as amended by said post-effective amendment.
          The term "Registration Statement" as used in this Agreement shall also
          include any registration statement relating to the Stock that is filed
          and declared effective pursuant to Rule 462(b) under the Securities
          Act. The term "Prospectus" as used in this Agreement means the
          prospectus in the form included in the Registration Statement, or, (A)
          if the prospectus included in the Registration Statement omits
          information in reliance on Rule 430A under the Securities Act and such
          information is included in a prospectus filed with the Commission
          pursuant to Rule 424(b) under the Securities Act, the term
          "Prospectus" as used in this Agreement means the prospectus in the
          form included in the Registration Statement as supplemented by the
          addition of the Rule 430A information contained in the prospectus
          filed with the Commission pursuant to Rule 424(b) and (B) if
          prospectuses that meet the requirements of Section 10(a) of the
          Securities Act are delivered pursuant to Rule 434 under the Securities
          Act, then (i) the term "Prospectus" as used in this Agreement means
          the "prospectus subject to completion" (as such term is defined in
          Rule 434(g) under the Securities Act) as supplemented by (a) the
          addition of Rule 430A information or other information contained in
          the form of prospectus delivered pursuant to Rule 434(b)(2) under the
          Securities Act or (b) the information contained in the term sheets
          described in Rule 434(b)(3) under the Securities Act, and (ii) the
          date of such prospectuses shall be deemed to be the date of the term
          sheets. The term



                                        2

<PAGE>   3



          "Preeffective Prospectus" as used in this Agreement means the
          prospectus subject to completion in the form included in the
          Registration Statement at the time of the initial filing of the
          Registration Statement with the Commission, and as such prospectus
          shall have been amended from time to time prior to the date of the
          Prospectus.

               (ii) The Commission has not issued or threatened to issue any
          order preventing or suspending the use of any Preeffective Prospectus,
          and, at its date of issue, each Preeffective Prospectus conformed in
          all material respects with the requirements of the Securities Act and
          did not include any untrue statement of a material fact or omit to
          state a material fact required to be stated therein or necessary to
          make the statements therein, in the light of the circumstances under
          which they were made, not misleading; and, when the Registration
          Statement becomes effective and at all times subsequent thereto up to
          and including the Closing Date (as hereinafter defined), the
          Registration Statement and the Prospectus and any amendments or
          supplements thereto contained and will contain all material statements
          and information required to be included therein by the Securities Act
          and conformed and will conform in all material respects to the
          requirements of the Securities Act and neither the Registration
          Statement nor the Prospectus, nor any amendment or supplement thereto,
          included or will include any untrue statement of a material fact or
          omit to state any material fact required to be stated therein or
          necessary to make the statements therein, in the light of the
          circumstances under which they were made, not misleading; provided,
          however, that the foregoing representations, warranties and agreements
          shall not apply to information contained in or omitted from any
          Preeffective Prospectus or the Registration Statement or the
          Prospectus or any such amendment or supplement thereto in reliance
          upon, and in conformity with, written information relating to any
          Underwriter (or relating to the distribution) furnished to the Company
          by or on behalf of any Underwriter, directly or through you,
          specifically for use in the preparation thereof; there is no
          franchise, lease, contract, agreement or document required to be
          described in the Registration Statement or Prospectus or to be filed
          as an


                                        3

<PAGE>   4



          exhibit to the Registration Statement which is not described or filed
          therein as required; and all descriptions of any such franchises,
          leases, contracts, agreements or documents contained in the
          Registration Statement are accurate and complete descriptions of such
          documents in all material respects.

               (iii) Subsequent to the respective dates as of which information
          is given in the Registration Statement and Prospectus, and except as
          set forth or contemplated in the Prospectus, neither the Company nor
          any of its subsidiaries has incurred any liabilities or obligations,
          direct or contingent, nor entered into any transactions not in the
          ordinary course of business, and there has not been any material
          adverse change in the condition (financial or otherwise), properties,
          business, management, prospects, net worth or results of operations of
          the Company and its subsidiaries considered as a whole, or any change
          in the capital stock, short-term or long-term debt of the Company and
          its subsidiaries considered as a whole.

               (iv) The financial statements, together with the related notes
          and schedules, set forth in the Prospectus and elsewhere in the
          Registration Statement fairly present, on the basis stated in the
          Registration Statement, the financial position and the results of
          operations and changes in financial position of the Company and its
          consolidated subsidiaries at the respective dates or for the
          respective periods therein specified. Such statements and related
          notes and schedules have been prepared in accordance with generally
          accepted accounting principles applied on a consistent basis except as
          may be set forth in the Prospectus. The other financial and
          statistical information and data set forth in the Prospectus and
          elsewhere in the Registration Statement is, in all material respects,
          accurately presented and, in the case of financial information and
          data, prepared on a basis consistent with such financial statements
          and the books and records of the Company.

               (v) Coopers & Lybrand L.L.P., who have expressed their opinions
          on the audited financial statements included in the Registration
          Statement and the Prospectus are independent public



                                        4

<PAGE>   5



          accountants as required by the Securities Act and the Rules and
          Regulations.

               (vi) Each of the Company, OraVax JVM, Inc., a Delaware
          corporation, and OraVax Securities Corp., a Massachusetts corporation
          (each of Oravax JVM, Inc. and OraVax Securities Corp., a
          "Subsidiary"), has been duly organized and is validly existing and in
          good standing as a corporation under the laws of its jurisdiction of
          organization, with power and authority (corporate and other) to own or
          lease its properties and to conduct its business as described in the
          Prospectus; and the Company is and the Subsidiaries are duly qualified
          to do business and in good standing as foreign corporations in all
          other jurisdictions where their ownership or leasing of properties or
          the conduct of their businesses requires such qualification, except
          where the failure to be so qualified would not have a material adverse
          effect on the Company and its subsidiaries, taken as a whole.

               (vii) Each of Merieux OraVax S.N.C. (the "French Partnership")
          and OraVax, Merieux Co. (the "U.S. Partnership") has been duly
          organized, is validly existing under the laws of its jurisdiction of
          organization and has full power and authority under the instrument
          pursuant to which it was formed, to carry on its business as described
          in the Prospectus and to own, lease and operate its properties, and is
          duly qualified and is in good standing as a foreign entity authorized
          to do business in each jurisdiction in which the nature of its
          business or its ownership or leasing of property requires such
          qualification, except where the failure to be so qualified would not
          have a material adverse effect on the Company and its subsidiaries,
          taken as a whole.

               (viii) The Company's authorized and outstanding capital stock is
          on the date hereof, and will be on the Closing Date, as set forth
          under the heading "Capitalization" in the Prospectus; the outstanding
          shares of Common Stock (including the outstanding shares of Stock) of
          the Company conform as to legal matters to the description thereof in
          the Prospectus and have been duly authorized and validly issued and
          are fully paid and nonassessable and have been issued in compliance
          with all federal and state


                                        5

<PAGE>   6



          securities laws and, on and after the Closing Date, will not be in
          violation of or subject to any preemptive rights or similar rights.
          There are no outstanding subscriptions, rights, warrants, options,
          calls, convertible securities, commitments of sale or liens related to
          or entitling any person to purchase or otherwise to acquire any shares
          of the capital stock of, or other ownership interest in, the Company,
          each Subsidiary, the U.S. Partnership or, to the knowledge of the
          Company, after due inquiry, the French Partnership except as issued
          upon the exercise of options or warrants described in the Registration
          Statement, grants or commitments under benefit plans described in the
          Registration Statement or as otherwise disclosed in the Registration
          Statement. All outstanding shares of capital stock of each Subsidiary
          have been duly authorized and validly issued, and are fully paid and
          nonassessable and are owned directly by the Company, and 50.1% of the
          ownership interests in the French Partnership and 49.9% of the
          ownership interests in the U.S. Partnership are owned by the Company,
          in each case, free and clear of any security interest, claim, lien,
          encumbrance or adverse interest of any nature.

               (ix) The Stock to be issued and sold by the Company to the
          Underwriters hereunder has been duly and validly authorized and, when
          issued and delivered against payment therefor as provided herein, will
          be duly and validly issued, fully paid and nonassessable and free of
          any preemptive or similar rights.

               (x) Except as set forth in the Prospectus, there are no material
          legal or governmental proceedings pending to which the Company or any
          of its subsidiaries is a party or of which any property of the Company
          or any subsidiary is subject; and to the best of the Company's
          knowledge no such proceedings are threatened or contemplated against
          the Company or any subsidiary by governmental authorities or others.
          The Company is not a party or subject to the provisions of any
          material injunction, judgment, decree or order of any court,
          regulatory body or other governmental agency or body. The description
          of the Company's litigation under the heading "Business--Legal
          Proceedings" in the



                                        6

<PAGE>   7



          Prospectus is true and correct and complies with the Rules and
          Regulations.

               (xi) Neither the Company nor any Subsidiary is in violation of
          its respective charter or by-laws or in default in the performance of
          any obligation, agreement or condition contained in any bond,
          debenture, note or any other evidence of indebtedness or in any other
          agreement, indenture or instrument material to the conduct of the
          business of the Company and its subsidiaries, taken as a whole, to
          which the Company or any of its subsidiaries is a party or by which it
          or any of its subsidiaries or their respective property is bound.

               (xii) The execution, delivery and performance of this Agreement
          and the consummation of the transactions herein contemplated will not
          result in a breach or violation of any of the terms or provisions of
          or constitute a default under any indenture, mortgage, deed of trust,
          note agreement or other agreement or instrument to which the Company
          or any of the subsidiaries is a party or by which it or any of its
          properties is or may be bound, the charter, by-laws or other
          organizational documents of the Company or any of its subsidiaries,
          except such breach or violation the effect of which would not be
          material to the Company and its subsidiaries, taken as a whole, or any
          law, order, rule or regulation of any court or governmental agency or
          body having jurisdiction over the Company or any of its subsidiaries
          or any of their properties or will result in the creation of a lien.

               (xiii) No consent, approval, authorization or order of any court
          or governmental agency or body is required for the consummation by the
          Company of the transactions contemplated by this Agreement, except
          such as may be required by the National Association of Securities
          Dealers, Inc. (the "NASD") or under the Securities Act or the
          securities or "Blue Sky" laws of any jurisdiction in connection with
          the purchase and distribution of the Stock by the Underwriters.

               (xiv) The Company has the full corporate power and authority to
          enter into this Agreement and to perform its obligations hereunder



                                        7

<PAGE>   8



          (including to issue, sell and deliver the Stock to be sold by it
          hereunder), and this Agreement has been duly and validly authorized,
          executed and delivered by the Company and is a valid and binding
          obligation of the Company, enforceable against the Company in
          accordance with its terms, except to the extent that rights to
          indemnity and contribution hereunder may be limited by federal or
          state securities laws or the public policy underlying such laws.

               (xv) The Company and its subsidiaries are in all material
          respects in compliance with, and conduct their businesses in
          conformity with, all applicable federal, state, local and foreign
          laws, rules and regulations of any court or governmental agency or
          body; to the knowledge of the Company, otherwise than as set forth in
          the Registration Statement and the Prospectus, no prospective change
          in any of such federal or state laws, rules or regulations has been
          adopted which, when made effective, would have a material adverse
          effect on the operations of the Company and its subsidiaries. The
          Company, each Subsidiary, the U.S. Partnership and to the knowledge of
          the Company, after due inquiry, the French Partnership (i) are in
          compliance with any and all applicable foreign, federal, state and
          local laws and regulations relating to the protection of human health
          and safety, the environment or hazardous or toxic substances or
          wastes, pollutants or contaminants ("Environmental Laws"), (ii) have
          received all permits, licenses or other approvals required of them
          under applicable Environmental Laws to conduct their respective
          businesses and (iii) are in compliance with all terms and conditions
          of any such permit, license or approval, except where such
          noncompliance with Environmental Laws, failure to receive required
          permits, licenses or other approvals or failure to comply with the
          terms and conditions of such permits, licenses or approvals would not,
          singly or in the aggregate, have a material adverse effect on the
          Company, its Subsidiaries, the U.S. Partnership and the French
          Partnership, taken as a whole.

               (xvi) The Company, each Subsidiary, the U.S. Partnership and to
          the knowledge of the Company, after due inquiry, the French
          Partnership have such permits, licenses, franchises and



                                        8

<PAGE>   9



          authorizations of governmental or regulatory authorities ("permits"),
          including, without limitation, under any applicable Environmental
          Laws, as are necessary to own, lease and operate their respective
          properties and to conduct their businesses except where such failure
          to receive required permits would not, singly or in the aggregate,
          have a material adverse effect on the Company, its Subsidiaries, the
          U.S. Partnership and the French Partnership, taken as a whole; the
          Company, each Subsidiary, the U.S. Partnership and, to the knowledge
          of the Company, after due inquiry, the French Partnership have
          fulfilled and performed all of their material obligations with respect
          to such permits and no event has occurred which allows, or after
          notice or lapse of time would allow, revocation or termination thereof
          or results in any other material impairment of the rights of the
          holder of any such permit; and, except as described in the Prospectus,
          such permits contain no restrictions that are materially burdensome to
          the Company or any of its subsidiaries.

               (xvii) In the ordinary course of its business, the Company
          internally conducts a periodic review of the effect of Environmental
          Laws on the business, operations and properties of the Company and its
          Subsidiaries, in the course of which it identifies and evaluates
          associated costs and liabilities (including, without limitation, any
          capital or operating expenditures required for clean-up, closure of
          properties or compliance with Environmental Laws or any permit,
          license or approval, any related constraints on operating activities
          and any potential liabilities to third parties). On the basis of such
          internally-conducted review, the Company has reasonably concluded that
          such associated costs and liabilities would not, singly or in the
          aggregate, have a material adverse effect on the Company, each
          Subsidiary, the French Partnership and the U.S. Partnership taken as a
          whole.

               (xviii) The Company and its subsidiaries have filed all material
          federal, state, local and foreign income, payroll, franchise and other
          tax returns, other than those filings being contested in good faith,
          and have paid all material taxes shown as due thereon or with respect
          to any of their properties, other



                                        9

<PAGE>   10



          than those being contested in good faith and for which adequate
          reserves have been provided, and there is no tax deficiency that has
          been, or to the knowledge of the Company is likely to be, asserted
          against the Company or any of its subsidiaries or any of their
          respective properties or assets that would materially adversely affect
          the financial position, business or operations of the Company and its
          subsidiaries.

               (xix) No person or entity has the right to require registration
          of shares of Common Stock or other securities of the Company because
          of the filing or effectiveness of the Registration Statement or
          otherwise, except as disclosed in the Prospectus.

               (xx) Neither the Company nor any of its officers, directors or
          affiliates has taken or will take, directly or indirectly, any action
          designed or intended to stabilize or manipulate the price of any
          security of the Company, or which caused or resulted in, or which
          might in the future reasonably be expected to cause or result in,
          stabilization or manipulation of the price of any security of the
          Company.

               (xxi) The Company is not involved in any labor dispute nor is any
          such dispute threatened. The Company is not aware that (A) any
          executive, key employee or significant group of employees of the
          Company or any subsidiary plans to terminate employment with the
          Company or any such subsidiary or (B) any such executive or key
          employee is subject to any noncompete, nondisclosure, confidentiality,
          employment, consulting or similar agreement that would be violated by
          the present or proposed business activities of the Company and its
          subsidiaries. Neither the Company nor any subsidiary has or expects to
          have any liability for any prohibited transaction or funding
          deficiency or any complete or partial withdrawal liability with
          respect to any pension, profit sharing or other plan which is subject
          to the Employee Retirement Income Security Act of 1974, as amended
          ("ERISA"), to which the Company or any subsidiary makes or ever
          has made a contribution and in which any employee of the Company or
          any subsidiary is or has ever been a participant. With respect to such
          plans, the Company and each subsidiary is in compliance in



                                       10

<PAGE>   11



          all material respects with all applicable provisions of ERISA.

               (xxii) The Company has obtained the written agreement described
          in Section 3 of this Agreement from each of its officers and directors
          who is not a Selling Stockholder, and each stockholder listed in
          Schedule C hereto.

               (xxiii) Except as otherwise disclosed in the Prospectus, the
          Company and its subsidiaries own or possess adequate licenses or other
          rights to use all material patents (or foreign equivalents),
          trademarks, licenses, copyrights and proprietary or other confidential
          information currently required by them in connection with their
          businesses except such as the failure to so own, possess or acquire
          would not have a material adverse effect on the Company. In connection
          with the filing of its patent applications, the Company conducted
          reasonable investigations of the published literature and patent
          references relating to the inventions claimed in such applications.
          There are no issued, enforceable United States or foreign patents
          known to the Company on the basis of a reasonable monitoring of
          patents issued in the United States which the Company believes to be
          infringed by its present activities or which would preclude the
          pursuit of its business as described in the Prospectus. The Company
          has not received any notice of infringement of or conflict with
          asserted rights of any third party with respect to any of the
          foregoing.

               (xxiv) The clinical trials and the animal studies conducted by or
          on behalf of the Company or in which the Company has participated that
          are described in the Prospectus were and, if still pending, are being
          conducted in accordance with standard medical and scientific research
          procedures, and the Company has operated and currently is in
          compliance in all material respects with all applicable FDA rules,
          regulations and policies.

               (xxv) No executive officer of the Company has received or is
          aware of any communication (written or oral) relating to the
          termination or modification of any of the agreements described or
          referred to in the



                                       11

<PAGE>   12



          Prospectus under the caption "Business--Technology Agreements", the
          termination or modification of which would have a material adverse
          effect on the Company.

               (xxvi) Except as disclosed in the Prospectus, there are no
          business relationships or related party transactions required to be
          disclosed therein by Item 404 of Regulation S-K of the Commission.

               (xxvii) Except as otherwise set forth in the Prospectus or such
          as are not material to the business, prospects, financial condition or
          results of operations of the Company, its Subsidiaries, the U.S.
          Partnership and the French Partnership, taken as a whole, the Company,
          each Subsidiary, the U.S. Partnership and, to the knowledge of the
          Company, after due inquiry, the French Partnership own or have valid
          rights to use all property and assets described in the Registration
          Statement as being owned by them. All leases to which the Company,
          each Subsidiary, the U.S. Partnership or, to the knowledge of the
          Company, after due inquiry, the French Partnership is a party are
          valid and binding and no default has occurred or is continuing
          thereunder, which might result in any material adverse change in the
          business, prospects, financial condition or results of operation of
          the Company, its Subsidiaries, the U.S. Partnership and the French
          Partnership, taken as a whole, and the Company, each Subsidiary, the
          U.S. Partnership and, to the knowledge of the Company, after due
          inquiry, the French Partnership enjoy quiet and undisturbed possession
          under all such leases to which any of them is a party as lessee with
          such exceptions as do not materially interfere with the use made by
          the Company, each Subsidiary, the French Partnership and the U.S.
          Partnership.

               (xxviii) The Company, each Subsidiary, the U.S. Partnership and,
          to the knowledge of the Company, after due inquiry, the French
          Partnership maintain reasonably adequate insurance.

               (xxix) The Company has complied with all provisions of Section
          517.075 Florida Statutes (Chapter 92-198; Laws of Florida).



                                       12

<PAGE>   13



               (xxx) The Company and each of its subsidiaries maintain a system
          of internal accounting controls sufficient to provide reasonable
          assurances that (A) transactions are executed in accordance with
          management's general or specific authorization; (B) transactions are
          recorded as necessary to permit preparation of financial statements in
          conformity with generally accepted accounting principles and to
          maintain accountability for assets; (C) access to assets is permitted
          only in accordance with management's general or specific
          authorization; and (D) the recorded accountability for assets is
          compared with existing assets at reasonable intervals and appropriate
          action is taken with respect to any differences.

               (xxxi) The Company is not an "investment company" or an entity
          "controlled" by an "investment company" as such terms are defined in
          the Investment Company Act of 1940, as amended.

               (xxxii) Other than as contemplated by this Agreement, there is no
          broker, finder or other party that is entitled to receive from the
          Company any brokerage or finder's fee or other fee or commission as a
          result of any of the transactions contemplated by this Agreement.

               (xxxiii) To the Company's knowledge, neither the Company nor any
          of its subsidiaries nor any employee or agent of the Company or any of
          its subsidiaries has made any payment of funds of the Company or any
          of its subsidiaries or received or retained any funds in violation of
          any law, rule or regulation, which payment, receipt or retention of
          funds is of a character required to be disclosed in the Prospectus.

               (xxxiv) Each certificate signed by any officer of the Company and
          delivered to the Underwriters or counsel for the Underwriters shall be
          deemed to be a representation and warranty by the Company as to the
          matters covered thereby.

          (b) REPRESENTATIONS AND WARRANTIES AND AGREEMENTS OF THE SELLING
STOCKHOLDERS. Each Selling Stockholder represents and warrants to, and agrees
with, the several Underwriters that such Selling Stockholder:



                                       13

<PAGE>   14



               (i) Now has, and on the Closing Date will have, valid and
          marketable title to the Stock to be sold by such Selling Stockholder,
          free and clear of any lien, claim, security interest or other
          encumbrance, including, without limitation, any restriction on
          transfer, and has full right, power and authority to enter into this
          Agreement, the Power of Attorney and the Custody Agreement (each as
          hereinafter defined).

               (ii) Now has, and on the Closing Date will have, upon delivery of
          and payment for each share of Stock hereunder, full right, power and
          authority, and any approval required by law to sell, transfer, assign
          and deliver the Stock being sold by such Selling Stockholder
          hereunder, and each of the several Underwriters will acquire valid and
          marketable title to all of the Stock being sold to the Underwriters by
          such Selling Stockholder, free and clear of any liens, encumbrances,
          equities, claims, restrictions on transfer or other defects
          whatsoever.

               (iii) Has duly executed and delivered a power of attorney, in
          substantially the form heretofore delivered by the Representatives
          (the "Power of Attorney"), appointing Lance K. Gordon and Keith S.
          Ehrlich, or either one of them, as attorney-in-fact (the
          "Attorneys-in-fact") with authority to execute and deliver this
          Agreement on behalf of such Selling Stockholder, to authorize the
          delivery of the shares of Stock to be sold by such Selling Stockholder
          hereunder and otherwise to act on behalf of such Selling Stockholder
          in connection with the transactions contemplated by this Agreement.

               (iv) Has duly executed and delivered a custody agreement, in
          substantially the form heretofore delivered by the Representatives
          (the "Custody Agreement"), with OraVax, Inc. as custodian (the
          "Custodian"), pursuant to which certificates in negotiable form for
          the shares of Stock to be sold by such Selling Stockholder hereunder
          have been placed in custody for delivery under this Agreement.

               (v) Has, by execution and delivery of each of this Agreement, the
          Power of Attorney and the Custody Agreement, created valid and binding
          obligations of such Selling Stockholder,



                                       14

<PAGE>   15



          enforceable against such Selling Stockholder in accordance with its
          terms, except to the extent that rights to indemnity hereunder may be
          limited by federal or state securities laws or the public policy
          underlying such laws.

               (vi) Has not taken, and will not take, directly or indirectly,
          any action designed to, or which might reasonably be expected to,
          cause or result in stabilization or manipulation of the price of any
          security of the Company to facilitate the sale or resale of the Stock
          pursuant to the distribution contemplated by this Agreement, and other
          than as permitted by the Securities Act, has not distributed and will
          not distribute any prospectus or other offering material in connection
          with the offering and sale of the Stock.

               (vii) The performance of this Agreement, the Custody Agreement
          and the Power of Attorney, and the consummation of the transactions
          contemplated hereby and thereby will not require any consent,
          approval, authorization or other order of any court, regulatory body,
          administrative agency or other governmental body (except as such may
          be required under the Securities Act, state securities laws or Blue
          Sky laws) and will not result in a breach or violation by such Selling
          Stockholder of any of the terms or provisions of, or constitute a
          default by such Selling Stockholder under, any indenture, mortgage,
          deed of trust, trust (constructive or other), loan agreement, lease,
          franchise, license or other agreement or instrument to which such
          Selling Stockholder is a party or by which such Selling Stockholder or
          any of its properties is bound, or any judgment of any court or
          governmental agency or body applicable to such Selling Stockholder or
          any of its properties, or to such Selling Stockholder's knowledge, any
          statute, decree, order, rule or regulation of any court or
          governmental agency or body applicable to such Selling Stockholder or
          any of its properties.

               (viii) Such parts of the Registration Statement under the caption
          "Principal and Selling Stockholders" which specifically relate to such
          Selling Stockholder do not, and will not on the Closing Date (and any
          Option Closing Date, if applicable), contain any untrue statement of a



                                       15

<PAGE>   16



          material fact or omit to state any material fact required to be stated
          therein or necessary to make the statements therein, in the light of
          the circumstances under which they were made, not misleading.

               (ix) At any time during the period described in paragraph 4(c)
          hereof, if there is any change in the information referred to in
          paragraph 7(viii) above, the Selling Stockholders will immediately
          notify you of such change.

          Each Selling Stockholder agrees that the shares of Stock represented 
by the certificates held in custody under the Custody Agreement are for the
benefit of and coupled with and subject to the interests of the Underwriters,
the other Selling Stockholders and the Company hereunder, and that the
arrangement for such custody and the appointment of the Attorneys-in-fact are
irrevocable; that the obligations of such Selling Stockholder hereunder shall
not be terminated by operation of law, whether by the death or incapacity,
liquidation or distribution of such Selling Stockholder, or any other event,
that if such Selling Stockholder should die or become incapacitated or is
liquidated or dissolved or any other event occurs, before the delivery of the
Stock hereunder, certificates for the Stock to be sold by such Selling
Stockholder shall be delivered on behalf of such Selling Stockholder in
accordance with the terms and conditions of this Agreement and the Custody
Agreement, and action taken by the Attorneys-in-fact or any of them under the
Power of Attorney shall be as valid as if such death, incapacity, liquidation or
dissolution or other event had not occurred, whether or not the Custodian, the
Attorneys-in-fact or any of them shall have notice of such death, incapacity,
liquidation or dissolution or other event.

          3. PURCHASE BY, AND SALE AND DELIVERY TO, UNDERWRITERS--CLOSING DATES.
The Company and the Selling Stockholders agree, severally and not jointly, to
sell to the Underwriters the Firm Stock, with the number of shares to be sold by
the Company being 2,500,000 shares, and the number of shares to be sold by each
Selling Stockholder being that number set opposite his or her name in Schedule
B; and on the basis of the representations, warranties, covenants and agreements
herein contained, but subject to the terms and conditions herein set forth, the
Underwriters agree, severally and not jointly, to purchase the Firm Stock from
the Company and the Selling Stockholders, the number of shares of Firm Stock to
be purchased by each Underwriter being set opposite its name in Schedule A,
subject to


                                       16

<PAGE>   17



adjustment in accordance with Section 12 hereof. The number of shares of Stock
to be purchased by each Underwriter from each Selling Stockholder hereunder
shall bear the same proportion to the total number of shares of Stock to be
purchased by such Underwriter hereunder as the number of shares of stock being
sold by each Selling Stockholder bears to the total number of shares of Stock
being sold by all Selling Stockholders, subject to adjustment by the
Representatives to eliminate fractions.

          The Company and the Selling Stockholders hereby agree, severally and 
not jointly, and the Company shall, concurrently with the execution of this
Agreement, deliver an agreement executed by (i) each of the directors and
officers of the Company who is not a Selling Stockholder, and (ii) each
stockholder listed in Schedule C hereto, pursuant to which each such person
agrees not, directly or indirectly, to offer, sell, contract to sell, grant any
option to purchase, or otherwise dispose of any Common Stock of the Company or
any securities convertible into or exercisable or exchangeable for such Common
Stock or cause to be filed with the Commission a registration statement under
the Securities Act, to register any shares of the Common Stock or, in any
manner, transfer all or a portion of the economic consequences associated with
the ownership of any such Common Stock, except to the Underwriters pursuant to
this Agreement, for a period of 90 days after the date of the Prospectus without
the prior written consent of Cowen. Notwithstanding the foregoing, during such
period (i) the Company may grant options and offer to sell shares of Common
Stock pursuant to the Company's benefit plans described in the Prospectus, (ii)
the Company may issue shares of its Common Stock upon the exercise of an option
or warrant or the conversion of a security outstanding on the date hereof and
(iii) the Company may issue up to 100,000 shares in respect of the acquisition
by the Company of the assets or capital stock of another person or entity of
which the Underwriters shall have been advised in writing, provided such shares
cannot be sold in the public market prior to the expiration of such 90 day
period.

          The purchase price per share to be paid by the Underwriters to the 
Company and the Selling Stockholders will be $ [ ] per share (the "Purchase
Price").

          The Company and the Selling Stockholders will deliver the Firm Stock 
to the Representatives for the respective accounts of the several Underwriters
in the form of definitive certificates, issued in such names and in such
denominations as the Representatives may direct by notice in writing to the
Company and the Selling Stockholders given at


                                       17

<PAGE>   18



or prior to 12:00 Noon, New York Time, on the second full business day preceding
the First Closing Date (as defined below) or, if no such direction is received,
in the names of the respective Underwriters or in such other names as Cowen may
designate (solely for the purpose of administrative convenience) and in such
denominations as Cowen may determine, against payment of the aggregate Purchase
Price therefor by wire of federal or other immediately available funds, payable
to the order of the Company and OraVax, Inc. as Custodian for the Selling
Stockholders, all at the offices of Hale and Dorr, 60 State Street, Boston,
Massachusetts 02109. The time and date of the delivery and closing shall be at
10:00 A.M., New York Time, on ________, 1996, in accordance with Rule 15c6-1 of
the Exchange Act. The time and date of such payment and delivery are herein
referred to as the "First Closing Date". The Closing Date and the location of
delivery of, and the form of payment for, the Firm Stock may be varied by
agreement among the Company, the Selling Stockholders and Cowen. The Closing
Date may be postponed pursuant to the provisions of Section 12.

          The Company and the Selling Stockholders shall make the certificates 
for the Stock available to the Representatives for examination on behalf of the
Underwriters not later than 10:00 A.M., New York Time, on the business day
preceding the First Closing Date at the offices of Cowen & Company, Financial
Square, New York, New York 10005.

          It is understood that Cowen, individually and not as Representative 
of the several Underwriters, may (but shall not be obligated to) make payment to
the Company or to the Selling Stockholders on behalf of any Underwriter or
Underwriters, for the Stock to be purchased by such Underwriter or Underwriters.
Any such payment by Cowen shall not relieve such Underwriter or Underwriters
from any of its or their other obligations hereunder.

          The several Underwriters agree to make an initial public offering of 
the Firm Stock at the initial public offering price as soon after the
effectiveness of the Registration Statement as in their judgment is advisable.
The Representatives shall promptly advise the Company and the Selling
Stockholders of the making of the initial public offering.

          For the purpose of covering any over-allotments in connection with the
distribution and sale of the Firm Stock as contemplated by the Prospectus, the
Company hereby grants to the Underwriters an option to purchase, severally and
not


                                       18

<PAGE>   19



jointly, up to the aggregate number of [381,705] shares of Optional Stock. The
price per share to be paid for the Optional Stock shall be the Purchase Price.
The option granted hereby may be exercised as to all or any part of the Optional
Stock at any time, and from time to time, not more than thirty (30) days
subsequent to the effective date of this Agreement. No Optional Stock shall be
sold and delivered unless the Firm Stock previously has been, or simultaneously
is, sold and delivered. The right to purchase the Optional Stock or any portion
thereof may be surrendered and terminated at any time upon notice by the
Underwriters to the Company.

          The option granted hereby may be exercised by the Underwriters by 
giving written notice from Cowen to the Company setting forth the number of
shares of the Optional Stock to be purchased by them and the date and time for
delivery of and payment for the Optional Stock. Each date and time for delivery
of and payment for the Optional Stock (which may be the First Closing Date, but
not earlier) is herein called the "Option Closing Date" and shall in no event be
earlier than two (2) business days nor later than ten (10) business days after
written notice is given. (The Option Closing Date and the First Closing Date are
herein called the "Closing Dates".) All purchases of Optional Stock from the
Company shall be made on a pro rata basis. Optional Stock shall be purchased for
the account of each Underwriter in the same proportion as the number of shares
of Firm Stock set forth opposite such Underwriter's name in Schedule A hereto
bears to the total number of shares of Firm Stock (subject to adjustment by the
Underwriters to eliminate odd lots). Upon exercise of the option by the
Underwriters, the Company agrees to sell to the Underwriters the number of
shares of Optional Stock set forth in the written notice of exercise and the
Underwriters agree, severally and not jointly and subject to the terms and
conditions herein set forth, to purchase the number of such shares determined as
aforesaid.

          The Company will deliver the Optional Stock to the Underwriters in the
form of definitive certificates, issued in such names and in such denominations
as the Representatives may direct by notice given at or prior to 12:00 Noon, New
York Time, on the second full business day preceding the Option Closing Date or,
if no such direction is received, in the names of the respective Underwriters or
in such other names as Cowen may designate (solely for the purpose of
administrative convenience) and in such denominations as Cowen may determine,
against payment of the aggregate Purchase Price therefor by certified or
official bank check or checks in Clearing House funds (next day


                                       19

<PAGE>   20



funds), payable to the order of the Company, all at the offices of Hale and
Dorr, 60 State Street, Boston, Massachusetts 02109. The Option Closing Date and
the location of delivery of, and the form of payment for, the Option Stock may
be varied by agreement between the Company and Cowen. The Option Closing Date
may be postponed pursuant to the provisions of Section 12.

          4. COVENANTS AND AGREEMENTS OF THE COMPANY. The Company covenants and
agrees with the several Underwriters that: 

               (a) The Company will (i) if the Company and the Representatives
          have determined not to proceed pursuant to Rule 430A, use its best
          efforts to cause the Registration Statement to become effective, (ii)
          if the Company and the Representatives have determined to proceed
          pursuant to Rule 430A, use its best efforts to comply with the
          provisions of and make all requisite filings with the Commission
          pursuant to Rule 430A and Rule 424 of the Rules and Regulations and
          (iii) if the Company and the Representatives have determined to
          deliver Prospectuses pursuant to Rule 434 of the Rules and
          Regulations, to use its best efforts to comply with all the applicable
          provisions thereof. The Company will advise the Representatives
          promptly as to the time at which the Registration Statement becomes
          effective, will advise the Representatives promptly of the issuance by
          the Commission of any stop order suspending the effectiveness of the
          Registration Statement or of the institution of any proceedings for
          that purpose, and will use its best efforts to prevent the issuance of
          any such stop order and to obtain as soon as possible the lifting
          thereof, if issued. The Company will advise the Representatives
          promptly of the receipt of any comments of the Commission or any
          request by the Commission for any amendment of or supplement to the
          Registration Statement or the Prospectus or for additional information
          and will not at any time file any amendment to the Registration
          Statement or supplement to the Prospectus which shall not previously
          have been submitted to the Representatives a reasonable time prior to
          the proposed filing thereof or to which the Representatives shall
          reasonably object in writing or which is not in compliance with the
          Securities Act and the Rules and Regulations.



                                       20

<PAGE>   21



               (b) The Company will prepare and file with the Commission,
          promptly upon the request of the Representatives, any amendments or
          supplements to the Registration Statement or the Prospectus which in
          the opinion of the Representatives may be necessary to enable the
          several Underwriters to continue the distribution of the Stock and
          will use its best efforts to cause the same to become effective as
          promptly as possible.

               (c) If at any time after the effective date of the Registration
          Statement when a prospectus relating to the Stock is required to be
          delivered under the Securities Act any event relating to or affecting
          the Company or any of its subsidiaries occurs as a result of which the
          Prospectus or any other prospectus as then in effect would include an
          untrue statement of a material fact, or omit to state any material
          fact necessary to make the statements therein, in the light of the
          circumstances under which they were made, not misleading, or if it is
          necessary at any time to amend the Prospectus to comply with the
          Securities Act, the Company will promptly notify the Representatives
          thereof and will prepare an amended or supplemented prospectus which
          will correct such statement or omission; and in case any Underwriter
          is required to deliver a prospectus relating to the Stock nine (9)
          months or more after the effective date of the Registration Statement,
          the Company upon the request of the Representatives and at the expense
          of such Underwriter will prepare promptly such prospectus or
          prospectuses as may be necessary to permit compliance with the
          requirements of Section 10(a)(3) of the Securities Act.

               (d) The Company will deliver to the Representatives, at or before
          the Closing Date, three (3) signed copies of the Registration
          Statement, as originally filed with the Commission, and all amendments
          thereto including all financial statements and exhibits thereto, and
          will deliver to the Representatives such number of copies of the
          Registration Statement, including such financial statements but
          without exhibits, and all amendments thereto, as the Representatives
          may reasonably request. The Company will deliver or mail to or upon
          the order of the Representatives, from time to time until the
          effective date of the Registration Statement, as


                                       21

<PAGE>   22



          many copies of the Preeffective Prospectus as the Representatives may
          reasonably request. The Company will deliver or mail to or upon the
          order of the Representatives on the date of the initial public
          offering, and thereafter from time to time during the period when
          delivery of a prospectus relating to the Stock is required under the
          Securities Act, as many copies of the Prospectus, in final form or as
          thereafter amended or supplemented as the Representatives may
          reasonably request; provided, however, that the expense of the
          preparation and delivery of any prospectus required for use nine (9)
          months or more after the effective date of the Registration Statement
          shall be borne by the Underwriters required to deliver such
          prospectus.

               (e) The Company will make generally available to its stockholders
          as soon as practicable, but not later than fifteen (15) months after
          the effective date of the Registration Statement, an earnings
          statement which will be in reasonable detail (but which need not be
          audited) and which will comply with Section 11(a) of the Securities
          Act, covering a period of at least twelve (12) months beginning after
          the "effective date" (as defined in Rule 158 under the Securities Act)
          of the Registration Statement.

               (f) The Company will cooperate with the Representatives to enable
          the Stock to be registered or qualified for offering and sale by the
          Underwriters and by dealers under the securities laws of such
          jurisdictions as the Representatives may designate and at the request
          of the Representatives will make such applications and furnish such
          consents to service of process or other documents as may be required
          of it as the issuer of the Stock for that purpose; provided, however,
          that the Company shall not be required to qualify to do business or to
          file a general consent (other than that arising out of the offering or
          sale of the Stock) to service of process in any such jurisdiction
          where it is not now so subject. The Company will, from time to time,
          prepare and file such statements and reports as are or may be required
          of it as the issuer of the Stock to continue such qualifications in
          effect for so long a period as the Representatives may reasonably
          request for the distribution of the Stock. The Company will advise the



                                       22

<PAGE>   23



          Representatives promptly after the Company becomes aware of the
          suspension of the qualifications or registration of (or any such
          exception relating to) the Common Stock of the Company for offering,
          sale or trading in any jurisdiction or of any initiation or threat of
          any proceeding for any such purpose, and in the event of the issuance
          of any orders suspending such qualifications, registration or
          exception, the Company will, with the cooperation of the
          Representatives, use its best efforts to obtain the withdrawal
          thereof.

               (g) The Company will furnish to its stockholders annual reports
          containing financial statements certified by independent public
          accountants and with quarterly summary financial information in
          reasonable detail which may be unaudited. During the period of five
          (5) years from the date hereof, the Company will deliver to the
          Representatives and, upon request, to each of the other Underwriters,
          as soon as they are available, copies of each annual report of the
          Company and each other report furnished by the Company to its
          stockholders and will deliver to the Representatives, (i) as soon as
          they are available, copies of any other reports (financial or other)
          which the Company shall publish or otherwise make available to any of
          its stockholders as such, (ii) as soon as they are available, copies
          of any reports and financial statements furnished to or filed with the
          Commission or any national securities exchange and (iii) from time to
          time such other information concerning the Company as you may request.
          So long as the Company has active subsidiaries, such financial
          statements will be on a consolidated basis to the extent the accounts
          of the Company and its subsidiaries are consolidated in reports
          furnished to its stockholders generally. Separate financial statements
          shall be furnished for all subsidiaries whose accounts are not
          consolidated but which at the time are significant subsidiaries as
          defined in the Rules and Regulations.

               (h) The Company will use its best efforts to maintain the
          inclusion of the Stock on the Nasdaq National Market (or on a national
          securities exchange) for a period of five (5) years after the
          effective date of the Registration Statement.



                                       23

<PAGE>   24



               (i) The Company will maintain a transfer agent and registrar for
          its Common Stock.

               (j) The Company will apply the net proceeds from the sale of the
          Stock as set forth in the description under "Use of Proceeds" in the
          Prospectus, which description complies in all respects with the
          requirements of Item 504 of Regulation S-K.

               (k) The Company will supply you with copies of all correspondence
          to and from, and all documents issued to and by, the Commission in
          connection with the registration of the Stock under the Securities
          Act.

               (l) Prior to the Closing Date the Company will furnish to you, as
          soon as they have been prepared, copies of any unaudited interim
          consolidated financial statements of the Company and its subsidiaries
          for any periods subsequent to the periods covered by the financial
          statements appearing in the Registration Statement and the Prospectus.

               (m) Prior to the Closing Date the Company will issue no press
          release or other communications directly or indirectly and hold no
          press conference with respect to the Company or any of its
          subsidiaries, the financial condition, results of operation, business,
          prospects, assets or liabilities of any of them, or the offering of
          the Stock, without your prior written consent.

               (n) The Company will use its best efforts to do and perform all
          things required or necessary to be done and performed under this
          Agreement by the Company prior to the Closing Date or any Option
          Closing Date, as the case may be, and to satisfy all conditions
          precedent to the delivery of the Stock.

          5. PAYMENT OF EXPENSES. (a) The Company will pay (directly or by
reimbursement) all costs, fees and expenses incurred in connection with expenses
incident to the performance of the obligations of the Company and of the Selling
Stockholders under this Agreement and in connection with the transactions
contemplated hereby, including but not limited to (i) all expenses and taxes
incident to the issuance and delivery of the Stock to the Representatives; (ii)
all expenses incident to the registration of the Stock



                                       24

<PAGE>   25



under the Securities Act; (iii) the costs of preparing stock certificates
(including printing and engraving costs); (iv) all fees and expenses of the
registrar and transfer agent of the Stock; (v) all necessary issue, transfer and
other stamp taxes in connection with the issuance and sale of the Stock to the
Underwriters; (vi) fees and expenses of the Company's counsel and the Company's
independent accountants; (vii) all costs and expenses incurred in connection
with the preparation, printing filing, shipping and distribution of the
Registration Statement, each Preeffective Prospectus and the Prospectus
(including all exhibits and financial statements) and all amendments and
supplements provided for herein, the Selling Stockholders' Powers of Attorney,
the Custody Agreement, the "Agreement Among Underwriters" between the
Representatives and the Underwriters, the Master Selected Dealers' Agreement,
the Underwriters' Questionnaire and the Blue Sky memoranda and this Agreement;
(viii) all filing fees, attorneys' fees and expenses incurred by the Company or
the Underwriters in connection with exemptions from the qualifying or
registering (or obtaining qualification or registration of) all or any part of
the Stock for offer and sale under the Blue Sky or other securities laws of such
jurisdictions as the Representatives may designate; (ix) all fees and expenses
paid or incurred in connection with filings made with the NASD; and (x) all
other costs and expenses incident to the performance of their obligations
hereunder which are not otherwise specifically provided for in this Section.

          6. INDEMNIFICATION AND CONTRIBUTION. (a) The Company agrees to 
indemnify and hold harmless each Underwriter and each person, if any, who
controls such Underwriter within the meaning of the Securities Act and the
respective officers, directors, partners, employees, representatives and agents
of each of such Underwriter (collectively, the "Underwriter Indemnified Parties"
and, each, an "Underwriter Indemnified Party"), against any losses, claims,
damages, liabilities or expenses (including the reasonable cost of investigating
and defending against any claims therefor and counsel fees incurred in
connection therewith), joint or several, which may be based upon the Securities
Act, or any other statute or at common law, on the ground or alleged ground that
any Preeffective Prospectus, the Registration Statement or the Prospectus (or
any Preeffective Prospectus, the Registration Statement or the Prospectus as
from time to time amended or supplemented) includes or allegedly includes an
untrue statement of a material fact or omits to state a material fact required
to be stated therein or necessary in order to make the statements therein, in
the light of the circumstances under which they were made, not misleading,
unless such statement


                                       25

<PAGE>   26



or omission was made in reliance upon, and in conformity with, written
information furnished to the Company by any Underwriter, directly or through the
Representatives, specifically for use in the preparation thereof; provided, that
with respect to any untrue statement or omission or alleged untrue statement or
omission made in any Preeffective Prospectus, the indemnity agreement contained
in this subsection (a) shall not inure to the benefit of any Underwriter
Indemnified Party from whom the person asserting any such losses, claims,
damages or liabilities purchased the shares of Stock concerned to the extent
that any such loss, claim, damage or liability of such Underwriter Indemnified
Party results from the fact that a copy of the Prospectus was not sent or given
to such person at or prior to the written confirmation of the sale of such
shares of Stock to such person as required by the Securities Act and if the
untrue statement or omission concerned has been corrected in the Prospectus. The
Company will be entitled to participate at its own expense in the defense or, if
it so elects, to assume the defense of any suit brought to enforce any such
liability, but if the Company elects to assume the defense, such defense shall
be conducted by counsel chosen by it. In the event the Company elects to assume
the defense of any such suit and retain such counsel, any Underwriter
Indemnified Parties, defendant or defendants in the suit, may retain additional
counsel but shall bear the fees and expenses of such counsel unless (i) the
Company shall have specifically authorized the retaining of such counsel or (ii)
the parties to such suit include any such Underwriter Indemnified Parties, and
the Company and such Underwriter Indemnified Parties at law or in equity have
been advised by counsel to the Underwriters that one or more legal defenses may
be available to it or them which may not be available to the Company, in which
case the Company shall not be entitled to assume the defense of such suit
notwithstanding its obligation to bear the fees and expenses of such counsel.
This indemnity agreement is not exclusive and will be in addition to any
liability which the Company might otherwise have and shall not limit any rights
or remedies which may otherwise be available at law or in equity to each
Underwriter Indemnified Party.

          (b) Each Selling Stockholder agrees to indemnify and hold harmless 
each Underwriter Indemnified Party against any losses, claims, damages,
liabilities or expenses (including, unless such Selling Stockholder elects to
assume the defense, the reasonable cost of investigating and defending against
any claims therefor and counsel fees incurred in connection therewith), joint or
several, which may be based upon the Securities Act, or any other statute or at
common law, on the ground or alleged ground that any


                                       26

<PAGE>   27



Preeffective Prospectus, the Registration Statement or the Prospectus (or any
Preeffective Prospectus, the Registration Statement or the Prospectus, as from
time to time amended and supplemented) includes an untrue statement of a
material fact or omits to state a material fact required to be stated therein or
necessary in order to make the statements therein, in the light of the
circumstances under which they were made, not misleading, unless such statement
or omission was made in reliance upon, and in conformity with, written
information furnished to the Company by any Underwriter, directly or through the
Representatives, specifically for use in the preparation thereof; provided, that
with respect to any untrue statement or omission or alleged untrue statement or
omission made in any Preeffective Prospectus, the indemnity agreement contained
in this subsection (b) shall not inure to the benefit of any Underwriter
Indemnified Party from whom the person asserting any such losses, claims,
damages or liabilities purchased the shares of Stock concerned to the extent
that any such loss, claim, damage or liability of such Underwriter Indemnified
Party results from the fact that a copy of the Prospectus was not sent or given
to such person at or prior to the written confirmation of the sale of such
shares of Stock to such person as required by the Securities Act and if the
untrue statement or omission concerned has been corrected in the Prospectus.
Such Selling Stockholder shall be entitled to participate at his or her own
expense in the defense, or, if he or she so elects, to assume the defense of any
suit brought to enforce any such liability, but, if such Selling Stockholder
elects to assume the defense, such defense shall be conducted by counsel chosen
by him or her. In the event that any Selling Stockholder elects to assume the
defense of any such suit and retain such counsel, the Underwriter Indemnified
Parties, defendant or defendants in the suit, may retain additional counsel but
shall bear the fees and expenses of such counsel unless (i) such Selling
Stockholder shall have specifically authorized the retaining of such counsel or
(ii) the parties to such suit include such Underwriter Indemnified Parties and
such Selling Stockholder and such Underwriter Indemnified Parties have been
advised by counsel that one or more legal defenses may be available to it or
them which may not be available to such Selling Stockholder, in which case such
Selling Stockholder shall not be entitled to assume the defense of such suit
notwithstanding its obligation to bear the fees and expenses of such counsel.
This indemnity agreement is not exclusive and will be in addition to any
liability which such Selling Stockholder might otherwise have and shall not
limit any rights or remedies which may otherwise be available at law or in
equity to each Underwriter Indemnified Party. The Company and the Selling
Stockholders may agree, as among


                                       27

<PAGE>   28



themselves and without limiting the rights of the Underwriters under this
Agreement, as to their respective amounts of such liability for which they each
shall be responsible. Notwithstanding any other provision of this Section 6, the
aggregate liability of any Selling Stockholder pursuant to the provisions of
this Section 6 shall be limited to an amount equal to the aggregate purchase
price received by such Selling Stockholder from the sale of such Selling
Stockholder's Stock hereunder.

          (c) Each Underwriter severally agrees to indemnify and hold harmless 
the Company, each of its directors, each of its officers who have signed the
Registration Statement and each person, if any, who controls the Company within
the meaning of the Securities Act (collectively, the "Company Indemnified
Parties") and each Selling Stockholder and each person, if any, who controls a
Selling Stockholder within the meaning of the Securities Act (collectively, the
"Stockholder Indemnified Parties"), against any losses, claims, damages,
liabilities or expenses (including, unless the Underwriter or Underwriters elect
to assume the defense, the reasonable cost of investigating and defending
against any claims therefor and counsel fees incurred in connection therewith),
joint or several, which arise out of or are based in whole or in part upon the
Securities Act, the Exchange Act or any other federal, state, local or foreign
statute or regulation, or at common law, on the ground or alleged ground that
any Preeffective Prospectus, the Registration Statement or the Prospectus (or
any Preeffective Prospectus, the Registration Statement or the Prospectus, as
from time to time amended and supplemented) includes an untrue statement of a
material fact or omits to state a material fact required to be stated therein or
necessary in order to make the statements therein, in the light of the
circumstances in which they were made, not misleading, but only insofar as any
such statement or omission was made in reliance upon, and in conformity with,
written information furnished to the Company by such Underwriter, directly or
through the Representatives, specifically for use in the preparation thereof;
provided, however, that in no case is such Underwriter to be liable with respect
to any claims made against any Company Indemnified Party or Stockholder
Indemnified Party against whom the action is brought unless such Company
Indemnified Party or Stockholder Indemnified Party shall have notified such
Underwriter in writing within a reasonable time after the summons or other first
legal process giving information of the nature of the claim shall have been
served upon the Company Indemnified Party or Stockholder Indemnified Party, but
failure to notify such Underwriter of such claim shall not relieve it from any



                                       28

<PAGE>   29



liability which it may have to any Company Indemnified Party or Stockholder
Indemnified Party otherwise than on account of its indemnity agreement contained
in this paragraph. Such Underwriter shall be entitled to participate at its own
expense in the defense, or, if it so elects, to assume the defense of any suit
brought to enforce any such liability, but, if such Underwriter elects to assume
the defense, such defense shall be conducted by counsel chosen by it. In the
event that any Underwriter elects to assume the defense of any such suit and
retain such counsel, the Company Indemnified Parties or Stockholder Indemnified
Parties and any other Underwriter or Underwriters or controlling person or
persons, defendant or defendants in the suit, shall bear the fees and expenses
of any additional counsel retained by them, respectively. The Underwriter
against whom indemnity may be sought shall not be liable to indemnify any person
for any settlement of any such claim effected without such Underwriter's
consent. This indemnity agreement is not exclusive and will be in addition to
any liability which such Underwriter might otherwise have and shall not limit
any rights or remedies which may otherwise be available at law or in equity to
any Company Indemnified Party or Stockholder Indemnified Party.

          (d) If the indemnification provided for in this Section 6 is 
unavailable or insufficient to hold harmless an indemnified party under
subsection (a) or (b) or (c) above in respect of any losses, claims, damages,
liabilities or expenses (or actions in respect thereof) referred to herein, then
each indemnifying party shall contribute to the amount paid or payable by such
indemnified party as a result of such losses, claims, damages, liabilities or
expenses (or actions in respect thereof) in such proportion as is appropriate to
reflect the relative benefits received by the Company and the Selling
Stockholders on the one hand and the Underwriters on the other from the offering
of the Stock. If, however, the allocation provided by the immediately preceding
sentence is not permitted by applicable law, then each indemnifying party shall
contribute to such amount paid or payable by such indemnified party in such
proportion as is appropriate to reflect not only such relative benefits but also
the relative fault of the Company and the Selling Stockholders on the one hand
and the Underwriters on the other in connection with the statements or omissions
which resulted in such losses, claims, damages, liabilities or expenses (or
actions in respect thereof), as well as any other relevant equitable
considerations. The relative benefits received by the Company and the Selling
Stockholders on the one hand and the Underwriters on the other shall be deemed
to be in the same proportion as the total net proceeds from the offering (before
deducting


                                       29

<PAGE>   30



expenses) received by the Company and the Selling Stockholders bear to the total
underwriting discounts and commissions received by the Underwriters, in each
case as set forth in the table on the cover page of the Prospectus. The relative
fault shall be determined by reference to, among other things, whether the
untrue or alleged untrue statement of a material fact or the omission or alleged
omission to state a material fact relates to information supplied by the
Company, the Selling Stockholders or the Underwriters and the parties' relative
intent, knowledge, access to information and opportunity to correct or prevent
such statement or omission. The Company, the Selling Stockholders and the
Underwriters agree that it would not be just and equitable if contribution were
determined by pro rata allocation (even if the Underwriters were treated as one
entity for such purpose) or by any other method of allocation which does not
take account of the equitable considerations referred to above. The amount paid
or payable by an indemnified party as a result of the losses, claims, damages,
liabilities or expenses (or actions in respect thereof) referred to above shall
be deemed to include any legal or other expenses reasonably incurred by such
indemnified party in connection with investigating, defending, settling or
compromising any such claim. Notwithstanding the provisions of this subsection
(d), no Underwriter shall be required to contribute any amount in excess of the
amount by which the total price at which the shares of the Stock underwritten by
it and distributed to the public were offered to the public exceeds the amount
of any damages which such Underwriter has otherwise been required to pay by
reason of such untrue or alleged untrue statement or omission or alleged
omission. The Underwriters' obligations to contribute are several in proportion
to their respective underwriting obligations and not joint. No person guilty of
fraudulent misrepresentation (within the meaning of Section 11(f) of the
Securities Act) shall be entitled to contribution from any person who was not
guilty of such fraudulent misrepresentation.

          7. SURVIVAL OF INDEMNITIES, REPRESENTATIONS, WARRANTIES ETC. The 
respective indemnities, covenants, agreements, representations, warranties and
other statements of the Company, the Selling Stockholders and the several
Underwriters, as set forth in this Agreement or made by them respectively,
pursuant to this Agreement, shall remain in full force and effect, regardless of
any investigation made by or on behalf of any Underwriter, the Selling
Stockholders, the Company or any of its officers or directors or any controlling
person, and shall survive delivery of and payment for the Stock.



                                       30

<PAGE>   31



          8. CONDITIONS OF UNDERWRITERS' OBLIGATIONS. The respective obligations
of the several Underwriters hereunder shall be subject to the satisfaction of
each of the following conditions:

               (a) All the representations and warranties of the Company
          contained in this Agreement shall be true and correct on the Closing
          Date with the same force and effect as if made on and as of the
          Closing Date.

               (b) The Registration Statement shall have become effective and no
          stop order suspending the effectiveness thereof shall have been issued
          and no proceedings for that purpose shall have been initiated or, to
          the knowledge of the Company or the Representatives, shall be
          threatened by the Commission, and any request for additional
          information on the part of the Commission (to be included in the
          Registration Statement or the Prospectus or otherwise) shall have been
          complied with to the reasonable satisfaction of the Representatives.
          Any filings of the Prospectus, or any supplement thereto, required
          pursuant to Rule 424(b) or Rule 434 of the Rules and Regulations,
          shall have been made in the manner and within the time period required
          by Rule 424(b) and Rule 434 of the Rules and Regulations, as the case
          may be.

               (c) Since the date of execution and delivery of this Agreement,
          there shall not have been any material adverse change, or any
          development involving a prospective material adverse change, in the
          condition, financial or otherwise, or in the earnings, affairs or
          business prospects, whether or not arising in the ordinary course of
          business, of the Company and the subsidiaries, taken as a whole, from
          that set forth in the Prospectus on the date the Registration
          Statement is declared effective, (ii) since the date of execution and
          delivery of this Agreement there shall not have been any material
          adverse change, or any development involving a prospective material
          adverse change, in the capital stock or in the long-term debt of the
          Company from that set forth in the Registration Statement and
          Prospectus, and (iii) the Company and its subsidiaries shall have no
          liability or obligation, direct or contingent, which is material to
          the Company and its subsidiaries,



                                       31

<PAGE>   32



          taken as a whole, other than those reflected in the Registration
          Statement and the Prospectus.

               (d) All the representations and warranties of the Selling
          Stockholders contained in this Agreement shall be true and correct on
          the Closing Date with the same force and effect as if made on and as
          of the Closing Date and the Representatives shall have received a
          certificate to such effect, dated the Closing Date, from each Selling
          Stockholder executed by his or her Attorney-in-fact.

               (e) The Representatives shall have received a certificate, dated
          the Closing Date, signed by Lance K. Gordon and Keith S. Ehrlich, in
          their respective capacities as the President and Chief Executive
          Officer, and Chief Financial Officer and Vice-President of Finance and
          Administration of the Company, confirming the matters set forth in
          paragraphs (a), (b) and (c) of this Section 8.

               (f) At the time of execution of this Agreement, the
          Representatives shall have received from Coopers & Lybrand L.L.P.,
          independent certified public accountants, a letter, dated the date
          hereof, in form and substance satisfactory to the Underwriters.

               (g) The Representatives shall have received from Coopers &
          Lybrand L.L.P., independent certified public accountants, a letter,
          dated the Closing Date, to the effect that such accountants reaffirm,
          as of the Closing Date, and as though made on the Closing Date, the
          statements made in the letter furnished by such accountants pursuant
          to paragraph (f) of this Section 8.

               (h) The Representatives shall have received on the Closing Date
          an opinion (satisfactory to the Representatives and counsel for the
          Underwriters), dated the Closing Date, of Hale and Dorr, counsel for
          the Company and special counsel to the Selling Stockholders, to the
          effect that:

               (i) each of the Company and each Subsidiary has been duly
          incorporated, is validly existing as a corporation in good


                                       32

<PAGE>   33



          standing under the laws of its jurisdiction of incorporation and has
          the corporate power and authority required to carry on its business as
          it is currently being conducted and to own, lease and operate its
          properties described in the Prospectus;

               (ii) each of the Company and each Subsidiary is duly qualified
          and is in good standing as a foreign corporation authorized to do
          business in those jurisdictions set forth in such opinion, which are
          the only jurisdictions in which the failure to be so qualified would
          have a material adverse effect on the Company;

               (iii) the U.S. Partnership has been duly organized, is validly
          existing under the laws of its jurisdiction of organization and has
          full power and authority under the instrument pursuant to which it was
          formed to carry on its business as described in the Prospectus and to
          own, lease and operate its properties;

               (iv) all of the outstanding shares of capital stock of, or other
          ownership interests in, each Subsidiary have been duly and validly
          authorized and issued and are fully paid and non-assessable, and are
          owned by the Company, free and clear of any security interest, claim,
          lien, encumbrance or adverse interest of any nature;

               (v) all the outstanding shares of Common Stock (including the
          Stock to be sold by the Selling Stockholders) have been duly
          authorized and validly issued and are fully paid, non-assessable and
          are, to such counsel's knowledge, not subject to any preemptive or
          similar rights;

               (vi) the Stock to be issued and sold by the Company hereunder has
          been duly authorized, and when issued and delivered to the
          Underwriters against payment therefor as provided by this Agreement,
          will have been validly issued and will be fully paid and
          non-assessable, and the issuance of such Stock is not, to such
          counsel's knowledge, subject to any preemptive or similar rights;


                                       33

<PAGE>   34




               (vii) this Agreement has been duly authorized, executed and
          delivered by the Company and each of the Selling Stockholders and is a
          valid and binding agreement of the Company and each Selling
          Stockholder except as rights to indemnity and contribution may be
          limited by applicable law;

               (viii) the authorized capital stock of the Company, including the
          Common Stock, conforms as to legal matters to the description thereof
          contained in the Prospectus under the caption "Description of Capital
          Stock;

               (ix) the Registration Statement has become effective under the
          Securities Act, and, to such counsel's knowledge, no stop order
          suspending its effectiveness has been issued and no proceedings for
          that purpose are pending before or contemplated by the Commission;

               (x) the statements under the captions "Risk Factors--Shares
          Eligible for Future Sales; Registration Rights", "Business--Benefits
          of Mucosal Immunity--UreAB Oral Vaccine Against H.
          pylori--Collaboration for H. pylori Product with Merieux",
          "Management--Employment Agreements", "Management--Employee Stock
          Plans", "Management--401(k) Savings and Retirement Plan", "Certain
          Transactions", "Description of Capital Stock", "Shares Eligible for
          Future Sale" in the Prospectus and Items 14 and 15 in the Registration
          Statement in each case insofar as such statements constitute summaries
          of the legal matters or documents referred to therein, fairly present
          the information called for with respect to such legal matters;

               (xi) the execution, delivery and performance of this Agreement by
          the Company and each Selling Stockholder, compliance by the Company
          and each Selling Stockholder with all the provisions hereof and the
          consummation of the transactions contemplated hereby will not require
          any consent, approval, authorization or other order of any court,
          regulatory body, administrative agency



                                       34

<PAGE>   35



          or other governmental body (except such as have been obtained or made
          and are in full force and effect and as such may be required by the
          NASD or under the Securities Act or other securities or Blue Sky laws)
          and will not conflict with or constitute a breach of any of the terms
          or provisions of, or a default under, the charter or by-laws of the
          Company or any of its subsidiaries or any agreement, indenture or
          other instrument to which the Company or any of its subsidiaries or
          any Selling Stockholder is a party or by which the Company or any of
          its subsidiaries or their respective properties are bound listed as an
          exhibit to the Registration Statement, or violate or conflict with any
          laws, administrative regulations or rulings or court decrees
          applicable to the Company or any of its subsidiaries or any Selling
          Stockholder or their respective properties;

               (xii) such counsel does not know of any legal or governmental
          proceeding pending or threatened to which the Company or any of its
          subsidiaries is a party or to which any of their respective property
          is subject which is required to be described in the Registration
          Statement or the Prospectus and is not so described, or of any
          contract or other document which is required to be described in the
          Registration Statement or the Prospectus or is required to be filed as
          an exhibit to the Registration Statement which is not described or
          filed as required;

               (xiii) the Company is not an "investment company" or a company
          "controlled" by an "investment company" within the meaning of the
          Investment Company Act of 1940, as amended;

               (xiv) to the best of such counsel's knowledge, no holder of any
          security of the Company has any right to require registration of
          shares of Common Stock or any other security of the Company except as
          described in the Registration Statement;

               (xv) the Registration Statement and the Prospectus and any
          supplement or amendment thereto (except for financial statements and



                                       35

<PAGE>   36



          schedules and other financial and statistical information included
          therein, as to which no opinion need be expressed) comply as to form
          in all material respects with the Securities Act. In passing upon the
          form of such documents, such counsel need not have independently
          verified and is not passing upon, and may have necessarily assumed the
          correctness and completeness of, the statements made therein and takes
          no responsibility therefor.

               (xvi) the Custody Agreement has been duly authorized, executed
          and delivered by each Selling Stockholder and is a valid and binding
          agreement of such Selling Stockholder enforceable in accordance with
          its terms;

               (xvii) each Selling Stockholder has full legal right, power and
          authority, and any approval required by law (other than any approval
          imposed by the applicable state securities and Blue Sky laws) to sell,
          assign, transfer and deliver the Stock to be sold by him or her in the
          manner provided in this Agreement and the Custody Agreement;

               (xviii) to such counsel's knowledge after due enquiry, each
          Selling Stockholder has good and clear title to the certificates for
          the Stock to be sold by him or her and upon delivery thereof, pursuant
          hereto and payment therefor, assuming the Underwriters purchase in
          good faith without notice of adverse claim under Section 8-302 of the
          Uniform Commercial Code, good and clear title will pass to the
          Underwriters, severally, free of all restrictions on transfer, liens,
          encumbrances, security interests and claims whatsoever; and

               (xix) the Power of Attorney signed by each Selling Stockholder
          appointing Lance K. Gordon and Keith S. Ehrlich, or either of them, as
          his or her Attorney-in-fact to the extent set forth therein with
          regard to the transactions contemplated hereby and by the Registration
          Statement has been duly authorized, executed and delivered by or on
          behalf of each Selling Stockholder and is a valid and binding
          instrument of such Selling


                                       36

<PAGE>   37



          Stockholder enforceable in accordance with its terms, and, pursuant to
          such Powers of Attorney, each of the Selling Stockholders has
          authorized Lance K. Gordon and Keith S. Ehrlich, or either of them, to
          execute and deliver on their behalf this Agreement and any other
          document necessary or desirable in connection with transactions
          contemplated hereby and to deliver the Stock to be sold by them
          pursuant to this Agreement.

     In addition to the matters set forth above, such opinion shall also include
     a statement to the effect that nothing has come to the attention of such
     counsel which leads them to believe that the Registration Statement, as of
     the time it becomes effective under the Securities Act, contained an untrue
     statement of a material fact or omitted to state a material fact required
     to be stated therein or necessary to make the statements therein not
     misleading, or that the Prospectus or any amendment or supplement thereto,
     on the date it was filed pursuant to Rule 424(b) and the Registration
     Statement and the Prospectus, or any amendment or supplement thereto, as of
     the Closing Date, or any later date on which Optional Stock is to be
     purchased, as the case may be, contains an untrue statement of material
     fact or omits to state a material fact required to be stated therein or
     necessary to make the statements therein, in the light of the circumstances
     under which they are made, not misleading (except that such counsel need
     express no view as to financial statements and schedules and other
     financial and statistical information included therein). With respect to
     such statement, counsel may state that their belief is based upon the
     procedures set forth therein, but is without independent investigation and
     verification.

               The opinion of Hale and Dorr described in paragraph (h) above 
     shall be rendered to you at the request of the Company or one or more of
     the Selling Stockholders, as the case may be, and shall so state therein.

               In rendering such opinion such counsel may (i) rely as to matters
     of fact upon the representations and warranties of the Company contained
     herein, provided that such counsel has no reason to believe that such
     reliance is not


                                       37

<PAGE>   38



     reasonable and (ii) assume that the laws of the State of New York governing
     this Agreement are the same as the laws of the Commonwealth of
     Massachusetts.

               (i) You shall have received on the Closing Date an opinion, dated
     the Closing Date, of Davis Polk & Wardwell, counsel for the Underwriters,
     as to the matters referred to in clauses (vi), (vii) (but only with respect
     to the Company), (x) (but only with respect to the statements under the
     caption "Description of Capital Stock" and "Underwriting"), (xv) and the
     ante-penultimate paragraph of the foregoing paragraph (h). In giving such
     opinion with respect to the matters covered by such ante-penultimate
     paragraph such counsel may state that their opinion and belief are based
     upon their participation in the preparation of the Registration Statement
     and Prospectus and any amendments or supplements thereto and review and
     discussion of the contents thereof, but are without independent check or
     verification except as specified.


               (j) You shall have received on the Closing Date an opinion of 
     Fish & Richardson, patent counsel for the Company, dated the Closing Date,
     to the effect that:

               (i) based on the information brought to such counsel's attention
          by the Company with respect to the Company's investigation, if any, of
          the published literature and patent references relating to the
          inventions claimed in its patent applications, such counsel disclosed
          all references known to it to the U.S. Patent and Trademark Office in
          accordance with 37 C.F.R. Section 1.56; to the best of such counsel's
          knowledge, all information submitted to the U.S. Patent and Trademark
          Office in the relevant applications, and in connection with the
          prosecution of the relevant applications, was accurate; neither such
          counsel, nor to the best of its knowledge, the Company, made any
          misrepresentation or concealed any material information from the U.S.
          Patent and Trademark Office in any of such applications, or in
          connection with the prosecution of such



                                       38

<PAGE>   39



          applications in violation of 37 C.F.R. Section 1.56;

               (ii) the statements in the Prospectus under the headings "Risk
          Factors -- Patents and Proprietary Rights" and "Business--Patents and
          Proprietary Rights; Technology Agreements", in each case insofar as
          such statements constitute summaries of the legal matters or documents
          referred to therein, fairly present the information called for with
          respect to such legal matters or documents; and

               (iii) other than as disclosed in the Prospectus, to the best of
          such counsel's knowledge, the Company has not received any notice of
          infringement of or conflict with asserted rights of any third party
          with respect to any material patents, trademarks, licenses, copyright
          and proprietary or other confidential information employed by the
          Company in connection with its business.

               In giving such opinion Fish & Richardson may state that they have
     relied upon the opinions of Lyon & Lyon and Nixon & Vanderhye P.C.,
     respectively, and may state that their opinion and belief are based upon
     their participation in the preparation of the Registration Statement and
     Prospectus and any amendments or supplements thereto and review and
     discussion of the contents thereof, but are without independent check or
     verification except as specified.

               (k) The Company shall have delivered to you the agreements 
     specified in Section 3 hereof.

               (l) The Company and the Selling Stockholders shall not have 
     failed at or prior to the Closing Date to perform or comply with any of the
     agreements herein contained and required to be performed or complied with
     by the Company at or prior to the Closing Date.

               (m) You shall have received on the Closing Date, a certificate 
     of each Selling Stockholder who is not a U.S. Person to the effect that
     such Selling Stockholder is not a U.S. Person (as defined under applicable
     U.S. federal tax legislation), which certificate may be in the form


                                       39

<PAGE>   40



     of a properly completed and executed United States Treasury Department Form
     W-8 (or other applicable form or statement specified by Treasury department
     regulations in lieu thereof).

     The several obligations of the Underwriters to purchase any Optional Stock
hereunder are subject to the delivery to you on the applicable Option Closing
Date of such documents as you may reasonably request with respect to the good
standing of the Company, the due authorization and issuance of such Optional
Stock and other matters related to the issuance of such Optional Stock.

     All opinions, certificates, letters and other documents will be in
compliance with the provisions hereunder only if they are satisfactory in form
and substance to the Representatives. The Company will furnish to the
Representatives conformed copies of such opinions, certificates, letters and
other documents as the Representatives shall reasonably request. If any of the
conditions hereinabove provided for in this Section shall not have been
satisfied when and as required by this Agreement, this Agreement may be
terminated by the Representatives by notifying the Company of such termination
in writing or by telegram at or prior to the Closing Date, but Cowen shall be
entitled to waive any of such conditions.

     9. EFFECTIVE DATE. This Agreement shall become effective immediately as to
Sections 5, 6, 7, 9, 10, 11, 13, 14, 15, 16, 17 and 18 and, as to all other
provisions, at 11:00 a.m. New York City time on the first full business day
following the effectiveness of the Registration Statement or at such earlier
time after the Registration Statement becomes effective as the Representatives
may determine on and by notice to the Company or by release of any of the Stock
for sale to the public. For the purposes of this Section 9, the Stock shall be
deemed to have been so released upon the release for publication of any
newspaper advertisement relating to the Stock or upon the release by you of
telegrams (i) advising Underwriters that the shares of Stock are released for
public offering or (ii) offering the Stock for sale to securities dealers,
whichever may occur first.

     10. TERMINATION. This Agreement (except for the provisions of Section 5)
may be terminated by the Company at any time before it becomes effective in
accordance with Section 9 by notice to the Representatives and may be terminated
by the Representatives at any time before it becomes effective in accordance
with Section 9 by notice to the Company. In the event of any termination of this



                                       40

<PAGE>   41



Agreement under this or any other provision of this Agreement, there shall be no
liability of any party to this Agreement to any other party, other than as
provided in Sections 5, 6 and 11 and other than as provided in Section 12 as to
the liability of defaulting Underwriters.

     This Agreement may be terminated after it becomes effective by the
Representatives by notice to the Company (i) if at or prior to the Closing Date
trading in securities on any of the New York Stock Exchange, American Stock
Exchange or the NASDAQ National Market shall have been suspended or minimum or
maximum prices shall have been established on any such exchange or market, or a
banking moratorium shall have been declared by New York or United States
authorities; (ii) trading of any securities of the Company shall have been
suspended on any exchange or in any over-the-counter market; (iii) if at or
prior to the Closing Date there shall have been (A) an outbreak or escalation of
hostilities between the United States and any foreign power or of any other
insurrection or armed conflict involving the United States or (B) any change in
financial markets or any calamity or crisis which, in the judgment of the
Representatives, makes it impractical or inadvisable to offer or sell the Stock
on the terms contemplated by the Prospectus; (iv) if there shall have been any
development or prospective development involving particularly the business or
properties or securities of the Company or any of its subsidiaries or the
transactions contemplated by this Agreement, which, in the judgment of the
Representatives, makes it impracticable or inadvisable to offer or deliver the
Stock on the terms contemplated by the Prospectus; (v) if there shall be any
litigation or proceeding, pending or threatened, which, in the judgment of the
Representatives, makes it impracticable or inadvisable to offer or deliver the
Stock on the terms contemplated by the Prospectus; or (vi) if there shall have
occurred any of the events specified in the immediately preceding clauses
(i)-(v) together with any other such event that makes it, in the judgment of the
Representatives, impractical or inadvisable to offer or deliver the Stock on the
terms contemplated by the Prospectus.

     11. REIMBURSEMENT OF UNDERWRITERS. Notwithstanding any other provisions
hereof, if this Agreement shall not become effective by reason of any election
of the Company or the Selling Stockholders pursuant to the first paragraph of
Section 10 or shall be terminated by the Representatives by reason of any
failure or refusal on the part of the Company and the Selling Stockholders to
comply with the terms or to fulfill any of the conditions of this Agreement, the
Company will bear and pay the expenses


                                       41

<PAGE>   42



specified in Section 5 hereof and, in addition to the obligations of the Company
and the Selling Stockholders pursuant to Section 6 hereof, the Company will
reimburse the reasonable out-of-pocket expenses of the several Underwriters
(including reasonable fees and disbursements of counsel for the Underwriters)
incurred in connection with this Agreement and the proposed purchase of the
Stock, and promptly upon demand the Company will pay such amounts to you as
Representatives.

     12. SUBSTITUTION OF UNDERWRITERS. If any Underwriter or Underwriters shall
default in its or their obligations to purchase shares of Stock hereunder and
the aggregate number of shares which such defaulting Underwriter or Underwriters
agreed but failed to purchase does not exceed ten percent (10%) of the total
number of shares underwritten, the other Underwriters shall be obligated
severally, in proportion to their respective commitments hereunder, to purchase
the shares which such defaulting Underwriter or Underwriters agreed but failed
to purchase. If any Underwriter or Underwriters shall so default and the
aggregate number of shares with respect to which such default or defaults occur
is more than ten percent (10%) of the total number of shares underwritten and
arrangements satisfactory to the Representatives and the Company for the
purchase of such shares by other persons are not made within forty-eight (48)
hours after such default, this Agreement shall terminate.

     If the remaining Underwriters or substituted Underwriters are required
hereby or agree to take up all or part of the shares of Stock of a defaulting
Underwriter or Underwriters as provided in this Section 12, (i) the Company and
the Selling Stockholders shall have the right to postpone the Closing Date for a
period of not more than five (5) full business days in order that the Company
and the Selling Stockholders may effect whatever changes may thereby be made
necessary in the Registration Statement or the Prospectus, or in any other
documents or arrangements, and the Company agrees promptly to file any
amendments to the Registration Statement or supplements to the Prospectus which
may thereby be made necessary, and (ii) the respective numbers of shares to be
purchased by the remaining Underwriters or substituted Underwriters shall be
taken as the basis of their underwriting obligation for all purposes of this
Agreement. Nothing herein contained shall relieve any defaulting Underwriter of
its liability to the Company, the Selling Stockholders or the other Underwriters
for damages occasioned by its default hereunder. Any termination of this
Agreement pursuant to this Section 12 shall be without liability on the part of
any non-defaulting



                                       42

<PAGE>   43



Underwriter, the Selling Stockholders or the Company, except for expenses to be
paid or reimbursed pursuant to Section 5 and except for the provisions of
Section 6.

          13. AGREEMENTS OF THE SELLING STOCKHOLDERS. Each Selling Stockholder
severally agrees with you and the Company:

          (a) To pay or to cause to be paid all transfer taxes with respect to
     the Stock to be sold by such Selling Stockholder; and

          (b) To take all reasonable actions in cooperation with the Company and
     the Underwriters to cause the Registration Statement to become effective at
     the earliest possible time, to do and perform all things to be done and
     performed under this Agreement prior to the Closing Date and to satisfy all
     conditions precedent to the delivery of the Stock pursuant to this
     Agreement.

          14. NOTICES. All communications hereunder shall be in writing and, if
sent to the Underwriters shall be mailed, delivered or telegraphed and confirmed
to you, as their Representatives, c/o Cowen & Company at Financial Square, New
York, New York 10005 except that notices given to an Underwriter pursuant to
Section 6 hereof shall be sent to such Underwriter at the address furnished by
the Representatives, or, if sent to the Company, shall be mailed, delivered or
telegraphed and confirmed to OraVax, Inc., 38 Sidney Street, Cambridge,
Massachusetts 02139, copy to: Hale and Door, 60 State Street, Boston,
Massachusetts 02109, Attn: John M. Westcott, Jr., Esq., or, if sent to the
Selling Stockholders, shall be mailed, delivered or telegraphed and confirmed to
Lance K. Gordon and Keith S. Ehrlich, c/o OraVax, Inc., 38 Sidney Street,
Cambridge, Massachusetts 02139.

          15. SUCCESSORS. This Agreement shall inure to the benefit of and be 
binding upon the several Underwriters, the Company and the Selling Stockholders
and their respective successors and legal representatives. Nothing expressed or
mentioned in this Agreement is intended or shall be construed to give any person
other than the persons mentioned in the preceding sentence any legal or
equitable right, remedy or claim under or in respect of this Agreement, or any
provisions herein contained, this Agreement and all conditions and provisions
hereof being intended to be and being for the sole and exclusive benefit of such
persons and for the benefit of no other person; except that the representations,
warranties, covenants, agreements and indemnities of the Company and the Selling


                                       43

<PAGE>   44



Stockholders contained in this Agreement shall also be for the benefit of the
person or persons, if any, who control any Underwriter or Underwriters within
the meaning of Section 15 of the Securities Act or Section 20 of the Exchange
Act, and the indemnities of the several Underwriters shall also be for the
benefit of each director of the Company, each of its officers who has signed the
Registration Statement and the person or persons, if any, who control the
Company or any Selling Stockholder within the meaning of Section 15 of the
Securities Act or Section 20 of the Exchange Act.

          16. APPLICABLE LAW. This Agreement shall be governed by and construed
in accordance with the laws of the State of New York.

          17. AUTHORITY OF THE REPRESENTATIVES. In connection with this 
Agreement, you will act for and on behalf of the several Underwriters, and any
action taken under this Agreement by Cowen, as Representative, will be binding
on all the Underwriters; and any action taken under this Agreement by any of the
Attorneys-in-fact will be binding on all the Selling Stockholders.

          18. PARTIAL UNENFORCEABILITY. The invalidity or unenforceability of 
any Section, paragraph or provision of this Agreement shall not affect the
validity or enforceability of any other Section, paragraph or provision hereof.
If any Section, paragraph or provision of this Agreement is for any reason
determined to be invalid or unenforceable, there shall be deemed to be made such
minor changes (and only such minor changes) as are necessary to make it valid
and enforceable.

          19. GENERAL. This Agreement constitutes the entire agreement of the 
parties to this Agreement and supersedes all prior written or oral and all
contemporaneous oral agreements, understandings and negotiations with respect to
the subject matter hereof.

          In this Agreement, the masculine, feminine and neuter genders and the
singular and the plural include one another. The section headings in this
Agreement are for the convenience of the parties only and will not affect the
construction or interpretation of this Agreement. This Agreement may be amended
or modified, and the observance of any term of this Agreement may be waived,
only by a writing signed by the Company, the Selling Stockholders and the
Representatives.



                                       44

<PAGE>   45



          20. COUNTERPARTS. This Agreement may be signed in two (2) or more
counterparts, each of which shall be an original, with the same effect as if the
signatures thereto and hereto were upon the same instrument.

          Any person executing and delivering this Agreement as Attorney-in-fact
for a Selling Stockholder represents by so doing that he has been duly appointed
as Attorney-in-fact by such Selling Stockholder pursuant to a validly existing
and binding Power of Attorney which authorizes such Attorney-in-fact to take
such action.


                                       45

<PAGE>   46




          If the foregoing correctly sets forth our understanding, please 
indicate your acceptance thereof in the space provided below for that purpose,
whereupon this letter and your acceptance shall constitute a binding agreement
between us.

                                            Very truly yours,

                                            ORAVAX, INC.


                                            By:_______________________
                                               Name:
                                               Title:



                                            SELLING STOCKHOLDERS LISTED
                                            IN SCHEDULE B

                                            By: Attorney-in-fact


                                            By:________________________
                                                Attorney-in-fact



Accepted and delivered in
               as of the
date first above written.

COWEN & COMPANY
DONALDSON, LUFKIN & JENRETTE SECURITIES CORPORATION 
  Acting on their own behalf
  and as Representatives of the several 
  Underwriters referred to in the
  foregoing Agreement.

By: Cowen Incorporated,
    its general partner

      By:_____________________
         Title:



                                       46

<PAGE>   47



                                   SCHEDULE A

                                  Underwriters
                                  ------------



                                                            Number
                                                         of shares of
                                                          Firm Stock
                                                            to be
                      Name                                Purchased
                      ----                                ---------

Cowen & Company.................................

Donaldson, Lufkin & Jenrette
  Securities Corporation........................







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

Total...........................................    ===========       =========




<PAGE>   48



                                   SCHEDULE B

                              Selling Stockholders
                              --------------------
<TABLE>
<CAPTION>
                                                                    Number of
                                                                    ---------
                                                                    shares of
                                                                    ---------
                                                                   Firm Stock
                                                                   ----------
                       Name                                        to be Sold
                       ----                                        ----------

<S>                                                                 <C>
Lance K. Gordon............................................          2,000

Samuel K. Ackerman.........................................         13,400

Samuel K. Ackerman
(as custodian for Nathan
Joseph Ackerman under
Massachusetts Uniform Transfers
to Minors Act) ............................................          1,600

Keith S. Ehrlich...........................................          6,000

Thomas P. Monath...........................................         10,000

Robert B. Rombauer.........................................         10,000

Donna Rombauer.............................................          1,700




                                                                    ------
Total......................................................         44,700
                                                                    ======

</TABLE>



<PAGE>   49


                                   SCHEDULE C


                          Required Stockholder Lock-ups
                          -----------------------------




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