REPLIGEN CORP
8-K, 1999-05-17
BIOLOGICAL PRODUCTS, (NO DIAGNOSTIC SUBSTANCES)
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<PAGE>


                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
                                     -------

                                    FORM 8-K

                                 CURRENT REPORT
                     PURSUANT TO SECTION 13 OR 15(d) OF THE
                         SECURITIES EXCHANGE ACT OF 1934


        Date of report (Date of earliest event reported): April 30, 1999


                              REPLIGEN CORPORATION
- --------------------------------------------------------------------------------
             (Exact name of Registrant as specified in its charter)

                    117 FOURTH AVENUE, NEEDHAM, MASSACHUSETTS
- --------------------------------------------------------------------------------
                    (Address of principal executive offices)

                                      02494
- --------------------------------------------------------------------------------
                                   (Zip Code)

                                 (781) 449-9560
- --------------------------------------------------------------------------------
               Registrant's telephone number, including area code

                           NO CHANGE SINCE LAST REPORT
- --------------------------------------------------------------------------------
             (Former name or address, if changed since last report)


           DELAWARE                       000-14656              04-2729386
           --------                       ---------              ----------
(State or other jurisdiction of   (Commission file number)   (I.R.S. Employer
 incorporation or organization)                              Identification No.)



<PAGE>


ITEM 5.  OTHER EVENTS.

         On April 30, 1999 and May 14, 1999, Repligen Corporation entered into
binding agreements with certain investors to raise an aggregate of $9,000,000 in
a private placement of an aggregate of 3,600,000 shares of its Common Stock at a
price of $2.50 per share.

ITEM 7.  FINANCIAL STATEMENTS, PRO FORMA FINANCIAL INFORMATION AND EXHIBITS.  

         (a)      Financial Statements of Business Acquired:  None required.

         (b)      Pro Forma Financial Information:  None required.

         (c)      EXHIBITS:

<TABLE>
<CAPTION>

         Exhibit No.               Description
         -----------               -----------
         <S>                       <C>
         4.1                       Stock Purchase Agreement dated as of April
                                   30, 1999, by and among Repligen Corporation
                                   and Wellington Management Company, LLP, as
                                   Investment Advisor to the investors listed
                                   on Schedule I thereto.*

         4.2                       Stock Purchase Agreement dated as of May 14,
                                   1999, by and among Repligen Corporation and
                                   the investors listed on the Schedule I
                                   thereto.*

         99.1                      Press release of the Company dated May 17,
                                   1999.

</TABLE>

* The schedules and exhibits to the Stock Purchase Agreements, included as
Exhibits 4.1 and 4.2 hereto, are not being filed herewith. The Stock Purchase
Agreements filed herewith contain a list briefly identifying the contents of all
schedules and exhibits to such documents. The Company undertakes to furnish
supplementally a copy of any omitted schedule and exhibit to the Commission upon
request. Pursuant to Item 601 (b) (2) of Regulation S-K, set forth below is a
list of the omitted schedules and exhibits.

STOCK PURCHASE AGREEMENT (EXHIBIT 4.1 HERETO)
Exhibit A         Opinion of Testa, Hurwitz & Thibeault, LLP
Schedule I        List of Investors


STOCK PURCHASE AGREEMENT (EXHIBIT 4.2 HERETO)
Exhibit A         Opinion of Testa, Hurwitz & Thibeault, LLP
Schedule I        List of Investors



                                      -2-
<PAGE>


                                    SIGNATURE


Pursuant to the requirements of the Securities Exchange Act of 1934, the
registrant has duly caused this report to be signed on its behalf by the
undersigned thereunto duly authorized.


                                    REPLIGEN CORPORATION



                                    By:  /s/ Walter Herlihy
                                         -----------------------------
                                         Name:  Walter Herlihy
                                         -----------------------------
                                         Title: President & CEO
                                         -----------------------------




Date: May 17, 1999



                                      -3-
<PAGE>


                                  EXHIBIT INDEX

<TABLE>
<CAPTION>

Exhibit No.             Description
- -----------             -----------
<S>                     <C>
4.1                     Stock Purchase Agreement dated as of April 30, 1999,
                        by and among Repligen Corporation and Wellington
                        Management Company, LLP, as Investment Advisor to the
                        investors listed on Schedule I thereto.*

4.2                     Stock Purchase Agreement dated as of May 14, 1999, by
                        and among Repligen Corporation and the investors listed
                        on the Schedule I thereto.*

99.1                    Press release of the Company dated May 17, 1999.


</TABLE>

* The schedules and exhibits to the Stock Purchase Agreements, included as 
Exhibits 4.1 and 4.2 hereto, are not being filed herewith. The Stock Purchase 
Agreements filed herewith contain a list briefly identifying the contents of 
all schedules and exhibits to such documents. The Company undertakes to 
furnish supplementally a copy of any omitted schedule and exhibit to the 
Commission upon request. Pursuant to Item 601 (b) (2) of Regulation S-K, set 
forth below is a list of the omitted schedules and exhibits.

STOCK PURCHASE AGREEMENT (EXHIBIT 4.1 HERETO)
Exhibit A     Opinion of Testa, Hurwitz & Thibeault, LLP
Schedule I    List of Investors

STOCK PURCHASE AGREEMENT (EXHIBIT 4.2 HERETO)
Exhibit A     Opinion of Testa, Hurwitz & Thibeault, LLP
Schedule I    List of Investors






                                      -4-

<PAGE>


                                                                     Exhibit 4.1




                            STOCK PURCHASE AGREEMENT



         THIS STOCK PURCHASE AGREEMENT (this "Agreement") is made this 30th day
of April 1999 by and among Repligen Corporation, a Delaware corporation (the
"Company"), and the investors named in the attached SCHEDULE I (each an
"Investor" and collectively, the "Investors").

         WHEREAS, the Company desires to issue and sell to the Investors,
severally and not jointly, and the Investors desire, severally and not jointly,
to acquire an aggregate of 2,600,000 shares (the "Shares") of the Company's
Common Stock, par value $.01 per share ("Common Stock"), at a price per share of
$2.50, for an aggregate consideration of $6,500,000, all as set forth on
SCHEDULE I hereto;

         WHEREAS, the Company and the Investors desire to set forth certain
matters to which they have agreed relating to the Shares;

         NOW, THEREFORE, in consideration of the premises and the mutual
covenants contained in this Agreement, the parties agree as follows:

                  ARTICLE I -- ISSUANCE OF SECURITIES; CLOSING

         SECTION 1.1 AUTHORIZATION OF SHARES. Subject to the terms and
conditions of this Agreement, the Company has authorized the issuance of the
Shares pursuant to this Agreement.

         SECTION 1.2 PURCHASE AND SALE OF SHARES. Subject to the terms and
conditions of this Agreement and in reliance upon the representations and
warranties of the Company contained herein, each Investor agrees to purchase
from the Company, and the Company agrees to sell to such Investor, on the
Closing Date (as hereinafter defined) the number of Shares set forth opposite
such Investor's name on SCHEDULE I.

         SECTION 1.3 DELIVERY OF THE SHARES AT CLOSING. The completion of the
purchase and sale of the Shares (the "Closing") shall occur at a place and time
(the "Closing Date") to be specified by the Company, not later than 90 days
after the date the Registration Statement (as hereinafter defined) is filed with
the United States Securities and Exchange Commission (the "SEC") and of which
each Investor will be notified with at least three days' notice in advance by
the Company. At the Closing, which should occur immediately prior to the
Registration Statement becoming effective, the Company shall deliver to each
Investor one or more stock certificates representing the number of Shares set
forth on SCHEDULE I hereto with respect to such Investor, each such certificate
to be registered in the name of such Investor or, if so indicated on SCHEDULE I
hereto, in the name of a nominee designated by such Investor.

         The Company's obligation to issue the Shares to each Investor shall be
subject to the following conditions, any one or more of which may be waived by
the Company: (a) receipt by

<PAGE>

the Company of a certified or official bank check or wire transfer of funds in
the full amount of the purchase price for the Shares being purchased hereunder
by such Investor; and (b) the accuracy of the representations and warranties
made by such Investor as of the date hereof and the date of the Closing and the
fulfillment of those undertakings of such Investor to be fulfilled prior to the
Closing.

         Each Investor's obligation to purchase the Shares shall be subject to
the following conditions, any one or more of which may be waived by such
Investor: (a) the Company shall have (i) filed with the SEC a registration
statement (the "Registration Statement") within twenty (20) business days after
the date hereof, (ii) received an indication from the SEC that it has no further
comments with respect to the Registration Statement, and (iii) submitted an
acceleration request providing for the Registration Statement to be declared
effective (and received confirmation from the SEC that such acceleration shall
be issued pursuant thereto) at a time immediately following the Closing and on
or prior to the 90th day after the date of its filing (b) the accuracy of the
representations and warranties of the Company as of the date hereof and on the
date of the Closing and (c) the Investors shall have received an opinion of
Testa, Hurwitz & Thibeault, LLP, counsel for the Company, substantially in the
form of EXHIBIT A hereto. The Investors' obligations hereunder shall terminate,
without prejudice to their rights to claim damages, in the event that the
Closing does not occur on or before the 90th day after filing of the
Registration Statement (except to the extent such 90 day period is extended in
accordance with the terms hereof or as the parties may otherwise mutually
agree.)




                  ARTICLE II -- REPRESENTATIONS AND WARRANTIES
                                 OF THE COMPANY

         The Company hereby represents and warrants to the Investor that, as of
the date of this Agreement, the following are true and correct:

         SECTION 2.1 ORGANIZATION AND STANDING OF THE COMPANY. The Company is a
corporation duly organized, validly existing, and in good standing under the
laws of the State of Delaware. The Company has full corporate power and
authority to enter into, deliver, and perform its obligations and undertakings
under this Agreement. The Company is duly authorized to conduct business and is
in good standing under the laws of each jurisdiction where such qualification is
required, except where the lack of such qualification would not have a material
adverse effect on the business, financial condition, operations, results of
operations, or future prospects of the Company. The Company has full corporate
power and authority to carry on the business in which it is engaged and to own
and use the properties owned and used by it.

         SECTION 2.2 CAPITALIZATION. The Company's entire authorized capital
stock consists of 30,000,000 shares of Common Stock, $.01 par value per share
the "Common Stock") and 5,000,000 shares of Preferred Stock, $.01 par value per
share (the "Preferred Stock"). As of March 31, 1999, there were outstanding
18,264,285 shares of Common Stock and no shares of

                                      -2-
<PAGE>

Preferred Stock. All such outstanding shares are validly issued, fully paid, and
non-assessable. Other than as indicated in the SEC Reports (as hereinafter
defined), the Company does not have outstanding any option, warrant, purchase
right, subscription right, stock appreciation right, phantom stock right, profit
participation right, agreement, or other commitment to issue or to acquire any
shares of its capital stock, or any securities or obligations convertible into
or exchangeable for its capital stock, and the Company has not given any person
any right to acquire from the Company or sell to the Company any shares of its
capital stock. There are no voting trusts, proxies, or other agreements or
understandings with respect to the voting of the capital stock of the Company.

         SECTION 2.3 VALIDITY OF THIS AGREEMENT. The execution and delivery by
the Company of this Agreement and the performance by the Company of its
obligations hereunder, and the issue, sale, and delivery of the Shares, have
been duly authorized and approved by all necessary corporate action. This
Agreement has been duly executed and delivered by the Company and constitutes a
valid and binding obligation of the Company, enforceable in accordance with its
terms. The execution and delivery by the Company of this Agreement and the
performance by the Company of its obligations hereunder and the issuance, sale,
and delivery of the Shares, will not (i) conflict with, or result in, any breach
of any of the terms of, or constitute a default under, the certificate of
incorporation or by-laws of the Company; or (ii) conflict with, result in a
breach of, constitute a default under, result in the acceleration of, create in
any party the right to accelerate, terminate, modify, or cancel or require any
notice under any agreement, instrument, covenant, or other restriction or
arrangement to which the Company is a party or by which it or any of its
properties or assets is bound.

         SECTION 2.4 GOVERNMENTAL CONSENT, ETC. Except for filings, consents,
permits, approvals, and authorizations, which will be obtained by the Company
prior to the Closing, no consent, approval, authorization, or other order of,
action by, filing with, or notification to any governmental authority is
required under existing law or regulation in connection with the execution,
delivery, and performance of the Agreement, or the offer, issue, sale or
delivery of the Shares pursuant to the Agreement, or the consummation of any
other transactions contemplated hereby.

         SECTION 2.5 VALID ISSUANCE OF SHARES. When issued and delivered against
payment therefor in accordance with the terms and conditions of this Agreement,
the Shares shall be (i) duly authorized and validly issued, fully paid and
non-assessable and (ii) not subject to any preemptive rights, liens, claims or
encumbrances, or other restrictions on transfer or other agreements or
understandings with respect in the voting of the Common Stock.

         SECTION 2.6 SEC REPORTS. The Common Stock is registered pursuant to
Section 12(g) of the Securities Exchange Act of 1934, as amended (the "Exchange
Act"). Copies of all reports filed by the Company with the SEC pursuant to the
Exchange Act during the period from March 31, 1998 to the date of this Agreement
(the "SEC Reports") have been furnished to the Investors. The Company has filed
in a timely manner all SEC Reports that the Company was required to file under
the Exchange Act during, from and after March 31, 1998. Such SEC Reports
complied in all material respects with the SEC's requirements as of their
respective

                                      -3-
<PAGE>

filing dates, and the information contained therein as of the date thereof did
not contain an 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 light of the circumstances under where they were made not misleading.

         SECTION 2.7 FINANCIAL STATEMENTS. The audited financial statements of
the Company contained in the Company's Annual Report on Form 10-K for the year
ended March 31, 1998, including the notes relating thereto, disclose all
material liabilities of the Company as of the date thereof. Such financial
statements, as well as the unaudited financial statements of the Company
contained in the Company's Form 10-Q for the period ended December 31, 1998,
including the notes relating thereto, have been prepared in accordance with
generally accepted accounting principles consistently applied throughout the
periods involved. Said financial statements and related notes fairly present the
financial position and the results of operations and cash flow of Company as of
the respective dates thereof and for the periods indicated. Since December 31,
1998, there has not been any material adverse change in the business, financial
condition, operations, results of operations, assets, employee relations,
customer or supplier relations or future prospects of the Company, except as
contemplated and set forth in the Company's Form 10-Q for the period ended
December 31, 1998 and other changes in the ordinary course of business.

         SECTION 2.8 NO VIOLATION. Neither the execution and delivery by the
Company of this Agreement, nor the consummation of the transactions contemplated
hereby or thereby, will violate any constitution, statute, rule, injunction,
judgment, order, decree, ruling, charge or other restriction of any government,
governmental agency, or court known to the Company to which the Company is
subject, or any provision of its certificate of incorporation or by-laws.

         SECTION 2.9 TITLE TO ASSETS. The Company has such title to its property
and such rights and franchises as are necessary to operate the Company in the
manner contemplated by this Agreement and as set forth in the SEC Reports.

         SECTION 2.10 SECURITIES LAWS. All notices, filings, registrations, or
qualifications under state securities or "blue sky" laws, which are required in
connection with the offer, issue, and delivery of the Shares pursuant to this
Agreement, have been, or will be, completed by the Company.

         SECTION 2.11 INTELLECTUAL PROPERTY. To the knowledge of the Company,
the Company owns or possesses sufficient rights to use all material patents,
patent rights, trademarks, copyrights, licenses, inventions, trade secrets,
trade names and know-how (collectively, "Intellectual Property") as are owned or
used by it or that are necessary for the conduct of its business as now
conducted except where the failure to currently own or possess could not
reasonably be expected to have a material adverse effect on the financial
condition, earnings, operations, prospects or business of the Company. The
Company has not received any notice of, nor has it any knowledge of, any
infringement of or conflict with asserted rights of the Company by others with
respect to any Intellectual Property, except as could not reasonably be


                                      -4-
<PAGE>

expected to have a material adverse effect on the financial condition or
earnings, operations, prospects or business of the Company.

         SECTION 2.12 REGULATORY COMPLIANCE. The Company currently operates its
business in conformity with all applicable laws, rules and regulations of each
jurisdiction in which the Company is conducting business, including, without
limitation, the United States Food and Drug Administration (the "FDA"), except
where the failure to be so in compliance would not have a material adverse
effect on the financial condition, earnings, operations or business prospects of
the Company and (ii) the Company has all necessary licenses, certificates,
authorizations, approvals, permits, franchises, orders and consents from all
state, federal and other governmental or regulatory authorities including,
without limitation, the FDA, which are necessary to the current conduct of its
business, except where the failure to be so in compliance or to have such
licenses would not have a material adverse effect on the financial condition,
earnings, operations or business prospects of the Company.

         SECTION 2.13 GENERAL COMPLIANCE. The Company is not in violation of its
certificate of incorporation, bylaws, or other organizational document, and the
Company is not in violation of any law, administrative regulation, ordinance or
order of any court or governmental agency, arbitration panel or authority
applicable to the Company, which violation, individually or in the aggregate,
would be reasonably likely to have a material adverse effect on the financial
condition or earnings, operations or business prospects of the Company.

         SECTION 2.14 LEGAL PROCEEDINGS. Except as otherwise set forth in the
Company's SEC Reports, there is no material legal or governmental proceeding
pending or, to the knowledge of the Company, threatened or contemplated to which
the Company is or may be a party or of which the business or property of the
Company may be subject which would have a material adverse effect upon the
financial condition, earnings, operations or business prospects of the Company.

                       ARTICLE III -- REPRESENTATIONS AND
                           WARRANTIES OF THE INVESTORS

         Each Investor severally and not jointly hereby acknowledges,
represents, warrants, and agrees as follows:

         SECTION 3.1 INVESTOR REPRESENTATIONS. The Investor represents and
warrants to, and covenants with, the Company that: (i) the Investor is an
"accredited investor" as defined in Regulation D under the Securities Act of
1933 (the "Securities Act") and the Investor is also knowledgeable,
sophisticated and experienced in making, and is qualified to make decisions with
respect to investments in shares presenting an investment decision like that
involved in the purchase of the Shares, including investments in comparable
companies, and has requested, received, reviewed and considered all information
it deemed relevant in making an informed decision to purchase the Shares; (ii)
the Investor is acquiring the number of Shares set forth on SCHEDULE I hereto in
the ordinary course of its business and for its own account for investment only
and with no present intention of distributing any of such Shares or any
arrangement or understanding with any other persons regarding the distribution
of such Shares; (iii) the Investor


                                      -5-
<PAGE>

will not, directly or indirectly, offer, sell, pledge, transfer or otherwise
dispose of (or solicit any offers to buy, purchase or otherwise acquire or take
a pledge of) any of the Shares except in compliance with the Securities Act,
applicable state securities laws and the respective rules and regulations
promulgated thereunder; and (iv) the Investor has, in connection with its
decision to purchase the number of Shares set forth on SCHEDULE I hereto, relied
only upon the representations and warranties of the Company contained herein.
Subject to Article IV herein, the Investor understands that its acquisition of
the Shares has not been registered under the Securities Act or registered or
qualified under any state securities law in reliance on specific exemptions
therefrom, which exemptions may depend upon, among other things, the bona fide
nature of the Investor's investment intent as expressed herein.

         SECTION 3.2 COMPLIANCE WITH SECURITIES LAWS. The Investor hereby
covenants with the Company not to make any sale of the Shares without complying
with the provisions of this Agreement, and without effectively causing the
prospectus delivery requirement under the Securities Act to be satisfied, and
the Investor acknowledges that the certificates evidencing the Shares will be
imprinted with a legend that prohibits their transfer except in accordance
therewith.

         SECTION 3.3 AUTHORITY OF INVESTOR; VALIDITY OF THIS AGREEMENT. The
Investor further represents and warrants to, and covenants with, the Company
that (i) the Investor has full right, power, authority and capacity to enter
into this Agreement and to consummate the transactions contemplated hereby and
has taken all necessary action to authorize the execution, delivery and
performance of this Agreement, and (ii) this Agreement constitutes a valid and
binding obligation of the Investor enforceable against the Investor in
accordance with its terms, except as enforceability may be limited by applicable
bankruptcy, insolvency, reorganization, moratorium or similar laws affecting
creditors' and contracting parties' rights generally and except as
enforceability may be subject to general principles of equity (regardless of
whether such enforceability is considered in a proceeding in equity or at law)
and except as the indemnification agreements of the Investor herein may be
legally unenforceable.

         SECTION 3.4 NO DISPOSITIONS. Investor will not, prior to the
effectiveness of the Registration Statement, sell, offer to sell, solicit offers
to buy, dispose of, loan, pledge or grant any right with respect to
(collectively, a "Disposition"), the Common Stock of the Company, nor will
Investor engage in any hedging or other transaction which is designed to or
could reasonably be expected to lead to or result in a Disposition of Common
Stock of the Company by the Investor or any other person or entity. Such
prohibited hedging or other transactions would include without limitation
effecting any short sale or having in effect any short position (whether or not
such sale or position is against the box and regardless of when such position
was entered into) or any purchase, sale or grant of any right (including without
limitation any put or call option) with respect to the Common Stock of the
Company or with respect to any security (other than a broad-based market basket
or index) that includes, relates to or derives any significant part of its value
from the Common Stock of the Company.

         SECTION 3.5 INVESTMENT DECISION BY INVESTOR. The Investor understands
that nothing in this Agreement or any other materials presented to the Investor
in connection with the

                                      -6-
<PAGE>


purchase and sale of the Shares constitutes legal, tax or investment advice. The
Investor has consulted such legal, tax and investment advisors as it, in its
sole discretion, has deemed necessary or appropriate in connection with its
purchase of Shares.

         SECTION 3.6 FOREIGN SALES. The Investor acknowledges, represents and
agrees that no action has been or will be taken in any jurisdiction outside the
United States by the Company that would permit an offering of the Shares, or
possession or distribution of offering materials in connection with the issue of
the Shares, in any jurisdiction outside the United States where action for that
purpose is required. Each Investor outside the United States will comply with
all applicable laws and regulations in each foreign jurisdiction in which it
purchases, offers, sells or delivers Shares or has in its possession or
distributes any offering material, in all cases at its own expense.

                    ARTICLE IV -- REGISTRATION OF THE SHARES;
                       COMPLIANCE WITH THE SECURITIES ACT

         SECTION 4.1 REGISTRATION PROCEDURES AND EXPENSES. The Company shall:

                   (a) subject to receipt of necessary information from the
Investors, prepare and file with the SEC, within twenty (20) business days after
the date hereof, the Registration Statement to enable the resale of the Shares
by the Investors from time to time through the automated quotation system of the
Nasdaq National Market (or such other exchange or trading market on which the
Shares are publicly traded, if applicable) or in privately-negotiated
transactions;

                   (b) use its best efforts, subject to receipt of necessary
information from the Investors, to cause the Registration Statement to become
effective within 90 days after the Registration Statement is filed by the
Company;

                   (c) prepare and file promptly with the SEC such amendments
and supplements to the Registration Statement and the Prospectus used in
connection therewith (a "Prospectus") as may be necessary to keep the
Registration Statement current and effective for a period not exceeding, with
respect to each Investor's Shares purchased hereunder, the earlier of (i) the
second anniversary of the Closing Date, (ii) the date on which such Investor may
sell all Shares then held by such Investor without restriction under Rule 144(k)
of the Securities Act, or (iii) such time as all Shares purchased by such
Investor hereunder have been sold pursuant to a registration statement.

                   (d) furnish to each Investor with respect to the Shares
registered under the Registration Statement such number of copies of
Prospectuses and preliminary Prospectuses (a "Preliminary Prospectus") in
conformity with the requirements of the Securities Act and such other documents
as such Investor may reasonably request promptly after receiving such request,
in order to facilitate the public sale or other disposition of all or any of the
Shares by such Investor, provided, however, that the obligation of the Company
to deliver copies of Prospectuses or Preliminary Prospectuses to such Investor
shall be subject to the receipt by the


                                      -7-
<PAGE>

Company of reasonable assurances from the Investor that such Investor will
comply with the applicable provisions of the Securities Act and of such other
securities or blue sky laws as may be applicable in connection with any use of
such Prospectuses or Preliminary Prospectuses;

                   (e) timely file documents required of the Company for blue
sky clearance in states specified in writing by each Investor, provided,
however, that the Company shall not be required to qualify to do business or
consent to service of process in any jurisdiction in which it is not now so
qualified or has not so consented;

                   (f) bear all expenses in connection with the procedures in
paragraph (a) through (e) of this Section 4.1 and the registration of the Shares
pursuant to the Registration Statement; and

                   (g) advise each Investor, promptly after it shall receive
notice or obtain knowledge of the issuance of any stop order by the SEC delaying
or suspending the effectiveness of the Registration Statement or of the
initiation or threat of any proceeding for that purpose; and it will promptly
use its best efforts to prevent the issuance of any stop order or to obtain its
withdrawal at the earliest possible moment if such stop order should be issued.

         The Company understands that each Investor disclaims being an
underwriter, but if such Investor is deemed an underwriter by the SEC, the
Company shall not be relieved of any obligations it has hereunder, PROVIDED,
HOWEVER that if the Company receives notification from the SEC that such
Investor is deemed an underwriter, then the period by which the Company is
obligated to submit an acceleration request to the SEC shall be extended for a
reasonable period not to exceed the earlier of (i) the 90th day after such SEC
notification, or (ii) 120 days after the initial filing of the Registration
Statement with the SEC.

         SECTION 4.2 TRANSFER OF SHARES AFTER REGISTRATION; SUSPENSION.

                   (a) Each Investor agrees that it will not effect any
disposition of the Shares or its right to purchase the Shares that would
constitute a sale within the meaning of the Securities Act except as
contemplated in the Registration Statement referred to in Section 4.1 and as
described below or under Rule 144 of the Securities Act, and that it will
promptly notify the Company of any changes in the information set forth in the
Registration Statement regarding the Investor or its plan of distribution of the
Shares.

                   (b) Except in the event that paragraph (c) below applies, the
Company shall (i) if deemed necessary by the Company, promptly prepare and file
from time to time with the SEC a post-effective amendment to the Registration
Statement or a supplement to the related Prospectus or a supplement or amendment
to any document incorporated therein by reference or file any other required
document so that such Registration Statement will not contain an 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 not misleading, and
so that, as thereafter delivered to purchasers of the Shares being sold
thereunder, such Prospectus will not contain an untrue statement of a material
fact or omit to state a material fact required to be stated therein or


                                      -8-
<PAGE>

necessary to make the statements therein, in light of the circumstances under
which they were made, not misleading; (ii) provide each Investor copies of any
documents filed pursuant to Section 4.2(b)(i); and (iii) inform each Investor
that the Company has complied with its obligations in Section 4.2(b)(i) (or
that, if the Company has filed a post-effective amendment to the Registration
Statement which has not yet been declared effective, the Company will notify
each Investor to that effect, will use its best efforts to secure the
effectiveness of such post-effective amendment as promptly as possible and will
promptly notify the Investor pursuant to Section 4.2(b)(i) hereof when the
amendment has become effective).

                   (c) Subject to paragraph (d) below, in the event (i) of any
request by the SEC or any other federal or state governmental authority during
the period of effectiveness of the Registration Statement for amendments or
supplements to a Registration Statement or related Prospectus or for additional
information; (ii) of the issuance by the SEC or any other federal or state
governmental authority of any stop order suspending the effectiveness of a
Registration Statement or the initiation of any proceedings for that purpose;
(iii) of the receipt by the Company of any notification with respect to the
suspension of the qualification or exemption from qualification of any of the
Shares for sale in any jurisdiction or the initiation or threatening of any
proceeding for such purpose; or (iv) of any event or circumstance which
necessitates the making of any changes in the Registration Statement or
Prospectus, or any document incorporated or deemed to be incorporated therein by
reference, so that, in the Registration Statement, it will not contain any
untrue statement of a material fact or any omission to state a material fact
required to be stated therein or necessary to make the statements therein not
misleading, and that in the case of the Prospectus, it will not contain any
untrue statement of a material fact or any omission 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; then
the Company shall deliver a certificate in writing to each Investor (the
"Suspension Notice") to the effect of the foregoing and, upon receipt of such
Suspension Notice, such Investor will refrain from selling any Shares pursuant
to the Registration Statement (a "Suspension") until the Investor's receipt of
copies of a supplemented or amended Prospectus prepared and filed by the
Company, or until it is advised in writing by the Company that the current
Prospectus may be used, and has received copies of any additional or
supplemental filings that are incorporated or deemed incorporated by reference
in any such Prospectus. In the event of any Suspension, the Company will use its
best efforts to cause the use of the Prospectus so suspended to be resumed as
soon as reasonably practicable within ten (10) business days after the delivery
of a Suspension Notice to each Investor. In addition to and without limiting any
other remedies (including, without limitation, at law or at equity), available
to each Investor, such Investor shall be entitled to specific performance in the
event that the Company fails to comply with the provisions of this Section
4.2(c).

                   (d) Notwithstanding the foregoing paragraphs of this Section
4.2, each Investor shall not be prohibited from selling Shares under the
Registration Statement as a result of Suspensions on more than two (2) occasions
of not more than fifteen (15) days each in any twelve month period.

                                      -9-
<PAGE>


                   (e) Provided that a Suspension is not then in effect, each
Investor may sell Shares under the Registration Statement, provided that such
Investor arranges for delivery of a current Prospectus to the transferee of such
Shares. Upon receipt of a request therefor, the Company has agreed promptly to
provide an adequate number of current Prospectuses to each Investor and to
supply copies to any other parties requiring such Prospectuses.

         SECTION 4.3 INDEMNIFICATION.  For the purpose of this Section 4.3 only:

               (i) the term "Selling Stockholder" shall include each Investor
and any affiliate of such Investor;

               (ii) the term "Registration Statement" shall include any final
Prospectus, exhibit, supplement or amendment included in or relating to the
Registration Statement referred to in Section 4.1; and

               (iii) the term "untrue statement" shall include any untrue
statement or alleged untrue statement, or any omission or alleged omission to
state in the Registration Statement 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.

                   (a) The Company agrees to indemnify and hold harmless each
Selling Stockholder from and against any losses, claims, damages or liabilities
to which such Selling Stockholder may become subject (under the Securities Act
or otherwise) insofar as such losses, claims, damages or liabilities (or actions
or proceedings in respect thereof) arise out of, or are based upon (i) any
untrue statement of a material fact contained in the Registration Statement, or
(ii) any failure by the Company to fulfill any undertaking included in the
Registration Statement, and the Company will reimburse such Selling Stockholder
for any reasonable legal or other expenses reasonably incurred in investigating,
defending or preparing to defend any such action, proceeding or claim, or
preparing to defend any such action, proceeding or claim, PROVIDED, HOWEVER,
that the Company shall not be liable in any such case to the extent that such
loss, claim, damage or liability arises out of, or is based upon, an untrue
statement made in such Registration Statement in reliance upon and in conformity
with written information furnished to the Company by or on behalf of such
Selling Stockholder specifically for use in preparation of the Registration
Statement or the failure of such Selling Stockholder to comply with its
covenants and agreements contained in Sections 3.2 or 4.2 hereof respecting sale
of the Shares or any statement or omission in any Prospectus that is corrected
in any subsequent Prospectus that was delivered to an Investor prior to the
pertinent sale or sales by such Investor.

                   (b) Each Investor agrees to indemnify and hold harmless the
Company (and each person, if any, who controls the Company within the meaning of
Section 15 of the Securities Act, each officer of the Company who signs the
Registration Statement and each director of the Company) from and against any
losses, claims, damages or liabilities to which the Company (or any such
officer, director or controlling person) may become subject (under the
Securities Act or otherwise), insofar as such losses, claims, damages or
liabilities (or actions or proceedings in respect thereof) arise out of, or are
based upon, (i) any failure to comply with the


                                      -10-
<PAGE>

covenants and agreements contained in Section 3.2 or 4.2 hereof respecting sale
of the Shares, or (ii) any untrue statement of a material fact contained in the
Registration Statement if such untrue statement was made in reliance upon and in
conformity with written information furnished by or on behalf of such Investor
specifically for use in preparation of the Registration Statement, and such
Investor will reimburse the Company (or such officer, director or controlling
person), as the case may be, for any legal or other expenses reasonably incurred
in investigating, defending or preparing to defend any such action, proceeding
or claim.

                   (c) Promptly after receipt by any indemnified person of a
notice of a claim or the beginning of any action in respect of which indemnity
is to be sought against an indemnifying person pursuant to this Section 4.3,
such indemnified person shall notify the indemnifying person in writing of such
claim or of the commencement of such action, but the omission to so notify the
indemnifying party will not relieve it from any liability which it may have to
any indemnified party under this Section 4.3 (except to the extent that such
omission materially and adversely affects the indemnifying party's ability to
defend such action) or from any liability otherwise than under this Section 4.3.
Subject to the provisions hereinafter stated, in case any such action shall be
brought against an indemnified person, the indemnifying person shall be entitled
to participate therein, and, to the extent that it shall elect by written notice
delivered to the indemnified party promptly after receiving the aforesaid notice
from such indemnified party, shall be entitled to assume the defense thereof,
with counsel reasonably satisfactory to such indemnified person. After notice
from the indemnifying person to such indemnified person of its election to
assume the defense thereof, such indemnifying person shall not be liable to such
indemnified person for any legal expenses subsequently incurred by such
indemnified person in connection with the defense thereof, PROVIDED, HOWEVER,
that if there exists or shall exist a conflict of interest that would make it
inappropriate, in the opinion of counsel to the indemnified person, for the same
counsel to represent both the indemnified person and such indemnifying person or
any affiliate or associate thereof, the indemnified person shall be entitled to
retain its own counsel at the expense of such indemnifying person; provided,
however, that no indemnifying person shall be responsible for the fees and
expenses of more than one separate counsel (together with appropriate local
counsel) for all indemnified parties. In no event shall any indemnifying person
be liable in respect of any amounts paid in settlement of any action unless the
indemnifying person shall have approved the terms of such settlement; PROVIDED
that such consent shall not be unreasonably withheld. No indemnifying person
shall, without the prior written consent of the indemnified person, effect any
settlement of any pending or threatened proceeding in respect of which any
indemnified person is or could have been a party and indemnification could have
been sought hereunder by such indemnified person, unless such settlement
includes an unconditional release of such indemnified person from all liability
on claims that are the subject matter of such proceeding.

                   (d) If the indemnification provided for in this Section 4.3
is unavailable to or insufficient to hold harmless an indemnified party under
subsection (a) or (b) above in respect of any losses, claims, damages or
liabilities (or actions or proceedings in respect thereof) referred to therein,
then each indemnifying party shall contribute to the amount paid or payable by
such indemnified party as a result of such losses, claims, damages or
liabilities (or actions in respect thereof) in such proportion as is appropriate
to reflect the relative fault of the


                                      -11-
<PAGE>

Company on the one hand and the Investors on the other in connection with the
statements or omissions or other matters which resulted in such losses, claims,
damages or liabilities (or actions in respect thereof), as well as any other
relevant equitable considerations. The relative fault shall be determined by
reference to, among other things, in the case of an untrue statement, whether
the untrue statement relates to information supplied by the Company on the one
hand or an Investor on the other and the parties' relative intent, knowledge,
access to information and opportunity to correct or prevent such untrue
statement. The Company and the Investors agree that it would not be just and
equitable if contribution pursuant to this subsection (d) were determined by pro
rata allocation (even if the Investors were treated as one entity for such
purpose) or by any other method of allocation which does not take into account
the equitable considerations referred to above in this subsection (d). The
amount paid or payable by an indemnified party as a result of the losses,
claims, damages or liabilities (or actions in respect thereof) referred to above
in this subsection (d) shall be deemed to include any legal or other expenses
reasonably incurred by such indemnified party in connection with investigating
or defending any such action or claim. Notwithstanding the provisions of this
subsection (d), no Investor shall be required to contribute any amount in excess
of the amount by which the net amount received by such Investor from the sale of
the Shares to which such loss relates exceeds the amount of any damages which
such Investor has otherwise been required to pay by reason of such untrue
statement. 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.

                   (e) The parties to this Agreement hereby acknowledge that
they are sophisticated business persons who were represented by counsel during
the negotiations regarding the provisions hereof including, without limitation,
the provisions of this Section 4.3, and are fully informed regarding said
provisions. They further acknowledge that the provisions of this Section 4.3
fairly allocate the risks in light of the ability of the parties to investigate
the Company and its business in order to assure that adequate disclosure is made
in the Registration Statement as required by the Securities Act and the Exchange
Act. The parties are advised that federal or state public policy as interpreted
by the courts in certain jurisdictions, may be contrary to certain of the
provisions of this Section 4.3, and the parties hereto hereby expressly waive
and relinquish any right or ability to assert such public policy as a defense to
a claim under this Section 4.3 and further agree not to attempt to assert any
such defense.

         SECTION 4.4 TERMINATION OF CONDITIONS AND OBLIGATIONS. The conditions
precedent imposed by Section 3 or this Section 4 upon the transferability of the
Shares shall cease and terminate as to any particular number of the Shares when
such Shares shall have been sold or otherwise disposed of in accordance with the
intended method of disposition set forth in the Registration Statement covering
such Shares or at such time as an opinion of counsel satisfactory to the Company
shall have been rendered to the effect that such conditions are not necessary in
order to comply with the Securities Act.

         SECTION 4.5 INFORMATION AVAILABLE. So long as the Registration
Statement is effective covering the resale of Shares owned by an Investor and
such Investor holds Shares subject to the Registration Statement, the Company
will furnish to such Investor:



                                      -12-
<PAGE>

                   (a) as soon as practicable after publicly available, one copy
of (i) its Annual Reports to Stockholders (which Annual Reports shall contain
financial statements audited in accordance with generally accepted accounting
principles by a national firm of certified public accountants), (ii) if not
included in substance in the Annual Reports to Stockholders, its Annual Reports
on Form 10-K, (iii) its Quarterly Reports on Form 10-Q, and (iv) a full copy of
the particular Registration Statement covering the Shares (the foregoing, in
each case, excluding exhibits); PROVIDED, HOWEVER, that in the event that the
Company is no longer required under the Exchange Act to file quarterly or annual
reports with the SEC, and for such time as the Company is not required to file
such reports (the "Non-Reporting Period"), the Company hereby agrees that it
shall use its best efforts to provide each Investor holding at least 70% of the
Shares purchased by such Investor hereunder (as adjusted for stock-splits,
combinations or other similar events) with audited financial statements within
120 days after the end of the respective fiscal year following commencement of
the Non-Reporting Period and unaudited financial statements of the Company for
the applicable quarterly period of the Company within 45 days after the end of
such fiscal period, following commencement of the Non-Reporting Period.

                   (b) upon the reasonable written request of such Investor, all
exhibits excluded by the parenthetical to subparagraph (a)(iv) of this Section
4.5 as filed with the SEC and all other information that is made available to
stockholders at no cost to such Investor; and

                   (c) upon the reasonable written request of such Investor, an
adequate number of copies of the Prospectuses to supply to any other party
requiring such Prospectuses; and the Company, upon the reasonable request of
such Investor, will meet with such Investor or a representative thereof at the
Company's headquarters to discuss all information relevant for disclosure in the
Registration Statement covering the Shares and will otherwise cooperate with any
Investor conducting an investigation for the purpose of reducing or eliminating
such Investor's exposure to liability under the Securities Act, including the
reasonable production of information at the Company's headquarters; provided,
that the Company shall not be required to disclose any confidential information
to or meet at its headquarters with any Investor until and unless such Investor
shall have entered into a confidentiality agreement in form and substance
reasonably satisfactory to the Company with the Company with respect thereto.

         SECTION 4.6 "LOCK-UP" AGREEMENT. If the Company proposes to offer for
sale to the public any of its equity securities, and (i) if an Investor is an
"affiliate" of the Company or otherwise holds beneficially or of record ten
percent (10%) or more of the outstanding equity securities of the Company, and
(ii) if requested by the Company and an underwriter of shares of Common Stock or
other securities of the Company and (iii) if all other "affiliates" and 10%
stockholders that purchased securities directly from the Company after the date
hereof pursuant to a private placement of securities have signed or are
contractually obligated to sign a lock-up agreement (as described below), then
such Investor shall not offer, sell, grant any option or right to buy or sell,
or otherwise transfer or dispose of in any manner any Common Stock or other
securities of the Company held by it during the 90-day or such shorter period
following the effective date of the registration statement of the Company filed
under the Securities Act and will

                                      -13-
<PAGE>

sign a "lock-up agreement" to such effect. Such agreement shall be in writing
and in form and substance reasonably satisfactory to the Company and such
underwriter and pursuant to customary and prevailing terms and conditions. The
Company may impose stop-transfer instructions with respect to the securities
subject to the foregoing restrictions until the end of such 90-day period.

         SECTION 4.7 NASDAQ LISTING. The Company shall use best efforts to
comply in all material respects with the requirements of the National
Association of Securities Dealers, Inc. with respect to the issuance of the
Shares and the listing thereof on the Nasdaq National Market.



                    ARTICLE V -- SURVIVAL AND INDEMNIFICATION

         SECTION 5.1 SURVIVAL. Notwithstanding any examination made by or on
behalf of any party hereto, the knowledge of any party or the acceptance by any
party of any certificate or opinion, each representation and warranty contained
herein shall survive the Closing and shall be fully effective and enforceable
for one year after the Closing, and each covenant contained herein shall survive
the Closing and shall be fully effective and enforceable.

         SECTION 5.2 INDEMNIFICATION.

                   (a) The Company shall indemnify each Investor, its
shareholders, officers, directors, employees, agents and representatives against
any damages, claims, losses, liabilities and expenses (including reasonable
counsel fees and expenses) which may be suffered or incurred by any of them as a
result of a breach of any representation, warranty or covenant made by the
Company in this Agreement;

                   (b) Each Investor agrees to indemnify the Company and its
shareholders, officers, directors, employees, agents and representatives against
any damages, claims, losses, liabilities and expenses (including reasonable
counsel fees and other expenses) which may be suffered or incurred by it as a
result of any breach of any representation, warranty, or covenant made by such
Investor in this Agreement; and

                   (c) In case any proceeding (including any governmental
investigation) shall be instituted involving any person in respect of which
indemnity may be sought pursuant to this Section, such person (the "indemnified
party") shall promptly notify the person against whom such indemnity may be
sought (the "indemnifying party") in writing of the occurrence of the facts and
circumstances giving rise to such claim. The failure of any person to deliver
the notice required by this Section 5.2(c) shall not in any way affect the
indemnifying party's indemnification obligations hereunder except and only to
the extent tat the indemnifying party is actually prejudiced thereby. In case
any such proceeding shall be brought against any indemnified party and it shall
notify the indemnifying party of the commencement thereof, the indemnifying
party shall be entitled to participate therein and, to the extent that it shall
which, jointly with any other indemnifying party similarly notified, to assume
the defense thereof, with counsel satisfactory to such indemnified party and
shall pay as incurred the fees and expenses of


                                      -14-
<PAGE>

such counsel related to such proceeding. In any such proceeding, any indemnified
party shall have the right to retain its own counsel or pay its own expenses.
Notwithstanding the foregoing, the indemnifying party shall pay as incurred the
fees and expenses of the counsel retained by the indemnified party in the event
(i) the indemnifying party and the indemnified party shall have mutually agreed
to the retention of such counsel or (ii) the named parties to any such
proceedings (including any impleaded parties) include both the indemnifying
party and the indemnified party and representations of both parties by the same
counsel would be inappropriate due to actual or potential differing interests
between them. The indemnifying party shall not be liable for any settlement of
any proceeding effected without its written consent (which shall not be
unreasonably withheld) but if settled with such consent or if there be a final
judgment for the plaintiff, the indemnifying party agrees to indemnify the
indemnified party from and against any loss or liability by reason of such
settlement or judgment.

                           ARTICLE VI -- MISCELLANEOUS

         SECTION 6.1 NOTICES. All notices, requests, consents and other
communications hereunder shall be in writing, shall be addressed to the
receiving party's address set forth below or to such other address as a party
may designated by notice hereunder, and shall be either (i) delivered by hand,
(ii) made by telex, telecopy or facsimile transmission, (iii) sent by overnight
courier, or (iv) sent by registered mail, return receipt requested, postage
prepaid.

         If to an Investor:  The address specified for such Investor on
                             SCHEDULE I.

         With a copy to:     Kirkpatrick & Lockhart, LLP
                             1800 Massachusetts Avenue, N.W.
                             Washington, D.C. 20036-1800
                             Attention: Thomas F. Cooney, III, Esq.
                             Facsimile: 202-778-9100

         and to:             Nicholas Holder, Esq.
                             Wellington Management Company
                             75 State Street
                             Boston, MA 02109
                             Facsimile: (617) 790-7760

         If to the Company:  Repligen Corporation
                             117 Fourth Avenue
                             Needham, MA  02194
                             Attention: Chief Executive Officer
                             Facsimile: 781-453-0048


                                      -15-
<PAGE>

         With a copy to:     Testa, Hurwitz & Thibeault, LLP
                             125 High Street
                             Boston, MA  02110
                             Attention: Lawrence S. Wittenberg, Esq.
                             Facsimile: 617-248-7100


All notices, requests, consents and other communications hereunder shall be
deemed to have been given together (i) if by hand, at the time of the delivery
thereof to the receiving party at the address of such party set forth above;
(ii) if by telex, telecopy or facsimile transmission, one (1) day after the time
that receipt thereof has been acknowledged by electronic confirmation or
otherwise; (iii) if sent by overnight courier, on the next business day
following the day such notice is delivered to the courier service; or (iv) if
sent by registered mail, on the 5th business day following the day such mailing
is made.

         SECTION 6.2 ENTIRE AGREEMENT. This Agreement, including exhibits, or
other documents referred to herein or that specifically indicate that they were
delivered to the Investors in connection with this Agreement, embodies the
entire agreement and understanding between the parties hereto with respect to
the subject matter hereof and supersedes all prior oral or written agreements
and understandings relating to the subject matter hereof. No statement,
representation, warranty, covenant or agreement of any kind not expressly set
forth in this Agreement or in any document that specifically indicates that it
was delivered to the Investors in connection with this Agreement shall affect,
or be used to interpret, change or restrict, the express terms and provisions of
this Agreement.

         SECTION 6.3 AMENDMENTS. The terms and provisions of the Agreement may
be modified, amended or waived, or consent for the departure therefrom granted,
only by written consent of the Company and the Investors holding at least 50% of
the Shares then held by all Investors. No such waiver or consent shall be deemed
to be an Agreement, whether or not similar. Each such waiver or consent shall be
effective only in the specific instance and for the purpose for which it was
given, and shall not constitute a continuing waiver or consent.

         SECTION 6.4 ASSIGNMENT. The rights and obligations under this Agreement
may not be assigned by any party hereto without the prior written consent of the
Company and Investors holding in the aggregate at least 50% of the Shares then
held by all Investors, which consent shall not be unreasonably withheld, except
that an Investor may transfer its rights and obligations to an "affiliate" (as
such term is defined under Rule 144 of the Securities Act) of such Investor.

         SECTION 6.5 BENEFIT. All statements, representations, warranties,
covenants and agreements in this Agreement shall be binding on the parties
hereto and shall inure to the benefit of the respective successors and permitted
assigns of each party hereto. Nothing in this Agreement shall be construed to
create any rights or obligations except among the parties hereto, and no person
or entity shall be regarded as a third-party beneficiary of this Agreement.


                                      -16-
<PAGE>

         SECTION 6.6 GOVERNING LAW. This Agreement and the rights and
obligations of the partied hereunder shall be construed in accordance with and
governed by the law of the Commonwealth of Massachusetts, without giving effect
to the conflict of law principles thereof.

         SECTION 6.7 SEVERABILITY. In the event that any court of competent
jurisdiction shall determine that any provision, or any portion thereof,
contained in this Agreement shall be unreasonable or unenforceable in any
respect, then such provision shall be deemed limited to the extent that such
court deems it reasonable and enforceable, and as so limited shall remain in
full force and effect. In the event that such court shall deem any such
provision, or portion thereof, wholly unenforceable, the remaining provisions of
this Agreement shall nevertheless remain in full force and effect.

         SECTION 6.8 HEADINGS AND CAPTIONS. The headings and captions of the
various subdivisions of this Agreement are for convenience of reference only and
shall in no way modify, or affect the meaning or constructions of any of the
terms or provisions hereof.

         SECTION 6.9 NO WAIVER OF RIGHTS, POWERS AND REMEDIES. No failure or
delay by a party hereto in exercising any right, power or remedy under this
Agreement, and no course of dealing between the parties hereto, shall operate as
a waiver of any such right, power or remedy of the party. No single or partial
exercise of any right, power or remedy under this Agreement by a party hereto,
nor any abandonment or discontinuance of steps to enforce any such right, power
or remedy, shall preclude such party from other or further exercise thereof or
the exercise of any other right, power or remedy hereunder. The election of any
remedy by a party hereto shall not constitute a waiver of the right of such
party to pursue other available remedies. No notice to or demand on a party not
expressly required under this Agreement shall entitle the party receiving such
notice or demand to any other or further notice or demand in similar or other
circumstances or constitute a waiver of the rights of the party giving such
notice or demand to any other or further action in any circumstances without
such notice or demand.

         SECTION 6.10 EXPENSES. Except as provided in Section 4.1(f), Section
4.3, Section 4.5(b) or Section 5.2, each of the parties shall pay its own fees
and expenses (including the fees of any attorneys, accountants, appraisers or
others engaged by such party) in connection with this Agreement and the
transactions contemplated hereby whether or not the transactions contemplated
hereby are consummated, except that the Company shall pay the legal fees and
expenses of the Investors' special counsel, Kirkpatrick & Lockhart LLP, in
connection with the negotiation and execution of this Agreement within thirty
(30) days after receipt of an invoice from Kirkpatrick & Lockhart LLP; provided
that the Company will not pay more than $10,000 for such fees and expenses.

         SECTION 6.11 BROKERS. Each of the parties hereto represents and
warrants to the other that no broker, finder or other financial consultant has
acted on its behalf in connection with this Agreement or the transactions
contemplated hereby in such a way as to create any liability on the other. Each
of the parties hereto agrees to indemnify and save the other harmless from any
claim or demand for commission, or for other compensation by any broker, finder,


                                      -17-
<PAGE>


financial consultant or similar agent claiming to have been employed by or on
behalf of such party and to bear the cost of legal expenses incurred in
defending against any such claim.

         SECTION 6.12 CONFIDENTIALITY. Each Investor acknowledges and agrees
that any information or data it has acquired from the Company, which is clearly
designated in writing as confidential and is not otherwise properly in the
public domain, was received in confidence. Each Investor agrees not to divulge,
communicate or disclose, except as may be required by law or for the performance
of this Agreement, or use to the detriment of the Company or for the benefit of
any other person or persons, or misuse in any way, any confidential information
of the Company.

         SECTION 6.13 COUNTERPARTS. This Agreement may be executed in one or
more counterparts, and by different parties hereto on separate counterparts,
each of which shall be deemed an original, but all of which together shall
constitute one and the same instrument.

         SECTION 6.14 FURTHER ASSISTANCE. In case at any time after the Closing
any further action is necessary or desirable to carry out the purposes of this
Agreement, the Company and each Investor will take such further action as the
other party may reasonably request, all at the sole cost and expense of the
requesting party (unless the requesting party is entitled to indemnification
under Article IV or V).



                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK.]


                                      -18-
<PAGE>


         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 30th day of April, 1999.

                         REPLIGEN CORPORATION



                         By: /s/ Walter C. Herlihy
                             -----------------------------------
                             Name:    Walter C. Herlihy
                             Title:   Chief Executive Officer


                             WELLINGTON MANAGEMENT COMPANY,
                             LLP, as Investment Adviser to the Investors named
                             in the Attached SCHEDULE I




                         By: /s/ Mary Ann Tynan
                             -----------------------------------
                             Name: Mary Ann Tynan
                             Title: Senior Vice President 


                                      -19-

<PAGE>


                         LIST OF EXHIBITS AND SCHEDULES


Exhibit A:   Opinion of Testa, Hurwitz & Thibeault, LLP

Schedule I:  List of Investors


                                      -1-


<PAGE>

                                                                    Exhibit 4.2


                            STOCK PURCHASE AGREEMENT



         THIS STOCK PURCHASE AGREEMENT (this "Agreement") is made this 14th day
of May 1999 by and among Repligen Corporation, a Delaware corporation (the
"Company"), and the investors named in the attached SCHEDULE I (each an
"Investor" and collectively, the "Investors").

         WHEREAS, the Company desires to issue and sell to the Investors,
severally and not jointly, and the Investors desire, severally and not jointly,
to acquire an aggregate of 1,000,000 shares (the "Shares") of the Company's
Common Stock, par value $.01 per share ("Common Stock"), at a price per share of
$2.50, for an aggregate consideration of $2,500,000, all as set forth on
SCHEDULE I hereto;

         WHEREAS, in addition to the Shares being issued and sold hereunder, the
Company has also entered into a Stock Purchase Agreement by and among the
Company and Wellington Management Company, LLP, as investment adviser to the
investors listed on Schedule I attached thereto (the "Wellington Stock Purchase
Agreement"), to issue and sell to those investors listed on Schedule I thereto
an aggregate of 2,600,000 shares of the Company's Common Stock, at a price per
share of $2.50 per share, for an aggregate consideration of $6,500,000, and on
such other substantially similar terms as set forth herein;

         WHEREAS, the Company and the Investors desire to set forth certain
matters to which they have agreed relating to the Shares;

         NOW, THEREFORE, in consideration of the premises and the mutual
covenants contained in this Agreement, the parties agree as follows:

                  ARTICLE I -- ISSUANCE OF SECURITIES; CLOSING

         SECTION 1.1 AUTHORIZATION OF SHARES. Subject to the terms and
conditions of this Agreement, the Company has authorized the issuance of the
Shares pursuant to this Agreement.

         SECTION 1.2 PURCHASE AND SALE OF SHARES. Subject to the terms and
conditions of this Agreement and in reliance upon the representations and
warranties of the Company contained herein, each Investor agrees to purchase
from the Company, and the Company agrees to sell to such Investor, on the
Closing Date (as hereinafter defined) the number of Shares set forth opposite
such Investor's name on SCHEDULE I.

         SECTION 1.3 DELIVERY OF THE SHARES AT CLOSING. The completion of the
purchase and sale of the Shares (the "Closing") shall occur at a place and time
(the "Closing Date") to be specified by the Company, not later than 90 days
after the date the Registration Statement (as hereinafter defined) is filed with
the United States Securities and Exchange Commission (the "SEC") and of which
each Investor will be notified with at least three days' notice in advance by
the Company. At the Closing, which should occur immediately prior to the
Registration


<PAGE>

Statement becoming effective, the Company shall deliver to each Investor one or
more stock certificates representing the number of Shares set forth on SCHEDULE
I hereto with respect to such Investor, each such certificate to be registered
in the name of such Investor or, if so indicated on SCHEDULE I hereto, in the
name of a nominee designated by such Investor.

         The Company's obligation to issue the Shares to each Investor shall be
subject to the following conditions, any one or more of which may be waived by
the Company: (a) receipt by the Company of a certified or official bank check or
wire transfer of funds in the full amount of the purchase price for the Shares
being purchased hereunder by such Investor; and (b) the accuracy of the
representations and warranties made by such Investor as of the date hereof and
the date of the Closing and the fulfillment of those undertakings of such
Investor to be fulfilled prior to the Closing.

         Each Investor's obligation to purchase the Shares shall be subject to
the following conditions, any one or more of which may be waived by such
Investor: (a) the Company shall have (i) filed with the SEC a registration
statement (the "Registration Statement") within twenty (20) business days after
the date hereof, (ii) received an indication from the SEC that it has no further
comments with respect to the Registration Statement, and (iii) submitted an
acceleration request providing for the Registration Statement to be declared
effective (and received confirmation from the SEC that such acceleration shall
be issued pursuant thereto) at a time immediately following the Closing and on
or prior to the 90th day after the date of its filing (b) the accuracy of the
representations and warranties of the Company as of the date hereof and on the
date of the Closing and (c) the Investors shall have received an opinion of
Testa, Hurwitz & Thibeault, LLP, counsel for the Company, substantially in the
form of EXHIBIT A hereto. The Investors' obligations hereunder shall terminate,
without prejudice to their rights to claim damages, in the event that the
Closing does not occur on or before the 90th day after filing of the
Registration Statement (except to the extent such 90 day period is extended in
accordance with the terms hereof or as the parties may otherwise mutually
agree.)




                  ARTICLE II -- REPRESENTATIONS AND WARRANTIES
                                 OF THE COMPANY

         The Company hereby represents and warrants to the Investor that, as of
the date of this Agreement, the following are true and correct:

         SECTION 2.1 ORGANIZATION AND STANDING OF THE COMPANY. The Company is a
corporation duly organized, validly existing, and in good standing under the
laws of the State of Delaware. The Company has full corporate power and
authority to enter into, deliver, and perform its obligations and undertakings
under this Agreement. The Company is duly authorized to conduct business and is
in good standing under the laws of each jurisdiction where such qualification is
required, except where the lack of such qualification would not have a material
adverse effect on the business, financial condition, operations, results of
operations, or future


                                      -2-
<PAGE>

prospects of the Company. The Company has full corporate power and authority to
carry on the business in which it is engaged and to own and use the properties
owned and used by it.

         SECTION 2.2 CAPITALIZATION. The Company's entire authorized capital
stock consists of 30,000,000 shares of Common Stock, $.01 par value per share
the "Common Stock") and 5,000,000 shares of Preferred Stock, $.01 par value per
share (the "Preferred Stock"). As of March 31, 1999, there were outstanding
18,264,285 shares of Common Stock and no shares of Preferred Stock. All such
outstanding shares are validly issued, fully paid, and non-assessable. Other
than as indicated in the SEC Reports (as hereinafter defined), and except for
the Wellington Stock Purchase Agreement, the Company does not have outstanding
any option, warrant, purchase right, subscription right, stock appreciation
right, phantom stock right, profit participation right, agreement, or other
commitment to issue or to acquire any shares of its capital stock, or any
securities or obligations convertible into or exchangeable for its capital
stock, and the Company has not given any person any right to acquire from the
Company or sell to the Company any shares of its capital stock. There are no
voting trusts, proxies, or other agreements or understandings with respect to
the voting of the capital stock of the Company.

         SECTION 2.3 VALIDITY OF THIS AGREEMENT. The execution and delivery by
the Company of this Agreement and the performance by the Company of its
obligations hereunder, and the issue, sale, and delivery of the Shares, have
been duly authorized and approved by all necessary corporate action. This
Agreement has been duly executed and delivered by the Company and constitutes a
valid and binding obligation of the Company, enforceable in accordance with its
terms. The execution and delivery by the Company of this Agreement and the
performance by the Company of its obligations hereunder and the issuance, sale,
and delivery of the Shares, will not (i) conflict with, or result in, any breach
of any of the terms of, or constitute a default under, the certificate of
incorporation or by-laws of the Company; or (ii) conflict with, result in a
breach of, constitute a default under, result in the acceleration of, create in
any party the right to accelerate, terminate, modify, or cancel or require any
notice under any agreement, instrument, covenant, or other restriction or
arrangement to which the Company is a party or by which it or any of its
properties or assets is bound.

         SECTION 2.4 GOVERNMENTAL CONSENT, ETC. Except for filings, consents,
permits, approvals, and authorizations, which will be obtained by the Company
prior to the Closing, no consent, approval, authorization, or other order of,
action by, filing with, or notification to any governmental authority is
required under existing law or regulation in connection with the execution,
delivery, and performance of the Agreement, or the offer, issue, sale or
delivery of the Shares pursuant to the Agreement, or the consummation of any
other transactions contemplated hereby.

         SECTION 2.5 VALID ISSUANCE OF SHARES. When issued and delivered against
payment therefor in accordance with the terms and conditions of this Agreement,
the Shares shall be (i) duly authorized and validly issued, fully paid and
non-assessable and (ii) not subject to any preemptive rights, liens, claims or
encumbrances, or other restrictions on transfer or other agreements or
understandings with respect in the voting of the Common Stock.



                                      -3-
<PAGE>


         SECTION 2.6 SEC REPORTS. The Common Stock is registered pursuant to
Section 12(g) of the Securities Exchange Act of 1934, as amended (the "Exchange
Act"). Copies of all reports filed by the Company with the SEC pursuant to the
Exchange Act during the period from March 31, 1998 to the date of this Agreement
(the "SEC Reports") have been furnished to the Investors. The Company has filed
in a timely manner all SEC Reports that the Company was required to file under
the Exchange Act during, from and after March 31, 1998. Such SEC Reports
complied in all material respects with the SEC's requirements as of their
respective filing dates, and the information contained therein as of the date
thereof did not contain an 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 light of the circumstances under where they were made not
misleading.

         SECTION 2.7 FINANCIAL STATEMENTS. The audited financial statements of
the Company contained in the Company's Annual Report on Form 10-K for the year
ended March 31, 1998, including the notes relating thereto, disclose all
material liabilities of the Company as of the date thereof. Such financial
statements, as well as the unaudited financial statements of the Company
contained in the Company's Form 10-Q for the period ended December 31, 1998,
including the notes relating thereto, have been prepared in accordance with
generally accepted accounting principles consistently applied throughout the
periods involved. Said financial statements and related notes fairly present the
financial position and the results of operations and cash flow of Company as of
the respective dates thereof and for the periods indicated. Since December 31,
1998, there has not been any material adverse change in the business, financial
condition, operations, results of operations, assets, employee relations,
customer or supplier relations or future prospects of the Company, except as
contemplated and set forth in the Company's Form 10-Q for the period ended
December 31, 1998 and other changes in the ordinary course of business.

         SECTION 2.8 NO VIOLATION. Neither the execution and delivery by the
Company of this Agreement, nor the consummation of the transactions contemplated
hereby or thereby, will violate any constitution, statute, rule, injunction,
judgment, order, decree, ruling, charge or other restriction of any government,
governmental agency, or court known to the Company to which the Company is
subject, or any provision of its certificate of incorporation or by-laws.

         SECTION 2.9 TITLE TO ASSETS. The Company has such title to its property
and such rights and franchises as are necessary to operate the Company in the
manner contemplated by this Agreement and as set forth in the SEC Reports.

         SECTION 2.10 SECURITIES LAWS. All notices, filings, registrations, or
qualifications under state securities or "blue sky" laws, which are required in
connection with the offer, issue, and delivery of the Shares pursuant to this
Agreement, have been, or will be, completed by the Company.

         SECTION 2.11 INTELLECTUAL PROPERTY. To the knowledge of the Company,
the Company owns or possesses sufficient rights to use all material patents,
patent rights, trademarks, copyrights, licenses, inventions, trade secrets,
trade names and know-how (collectively, "Intellectual Property") as are owned or
used by it or that are necessary for the



                                      -4-
<PAGE>


conduct of its business as now conducted except where the failure to currently
own or possess could not reasonably be expected to have a material adverse
effect on the financial condition, earnings, operations, prospects or business
of the Company. The Company has not received any notice of, nor has it any
knowledge of, any infringement of or conflict with asserted rights of the
Company by others with respect to any Intellectual Property, except as could not
reasonably be expected to have a material adverse effect on the financial
condition or earnings, operations, prospects or business of the Company.

         SECTION 2.12 REGULATORY COMPLIANCE. The Company currently operates its
business in conformity with all applicable laws, rules and regulations of each
jurisdiction in which the Company is conducting business, including, without
limitation, the United States Food and Drug Administration (the "FDA"), except
where the failure to be so in compliance would not have a material adverse
effect on the financial condition, earnings, operations or business prospects of
the Company and (ii) the Company has all necessary licenses, certificates,
authorizations, approvals, permits, franchises, orders and consents from all
state, federal and other governmental or regulatory authorities including,
without limitation, the FDA, which are necessary to the current conduct of its
business, except where the failure to be so in compliance or to have such
licenses would not have a material adverse effect on the financial condition,
earnings, operations or business prospects of the Company.

         SECTION 2.13 GENERAL COMPLIANCE. The Company is not in violation of its
certificate of incorporation, bylaws, or other organizational document, and the
Company is not in violation of any law, administrative regulation, ordinance or
order of any court or governmental agency, arbitration panel or authority
applicable to the Company, which violation, individually or in the aggregate,
would be reasonably likely to have a material adverse effect on the financial
condition or earnings, operations or business prospects of the Company.

         SECTION 2.14 LEGAL PROCEEDINGS. Except as otherwise set forth in the
Company's SEC Reports, there is no material legal or governmental proceeding
pending or, to the knowledge of the Company, threatened or contemplated to which
the Company is or may be a party or of which the business or property of the
Company may be subject which would have a material adverse effect upon the
financial condition, earnings, operations or business prospects of the Company.

                       ARTICLE III -- REPRESENTATIONS AND
                           WARRANTIES OF THE INVESTORS

          Each Investor severally and not jointly hereby acknowledges,
represents, warrants, and agrees as follows:


          SECTION 3.1 INVESTOR REPRESENTATIONS. The Investor represents and
warrants to, and covenants with, the Company that: (i) the Investor is an
"accredited investor" as defined in Regulation D under the Securities Act of
1933 (the "Securities Act") and the Investor is also knowledgeable,
sophisticated and experienced in making, and is qualified to make decisions with
respect to investments in shares presenting an investment decision like that
involved in the purchase of the Shares, including investments in comparable
companies, and has requested, received, reviewed and considered all information
it deemed relevant in making an informed


                                      -5-
<PAGE>


decision to purchase the Shares; (ii) the Investor is acquiring the number of
Shares set forth on SCHEDULE I hereto in the ordinary course of its business and
for its own account for investment only and with no present intention of
distributing any of such Shares or any arrangement or understanding with any
other persons regarding the distribution of such Shares; (iii) the Investor will
not, directly or indirectly, offer, sell, pledge, transfer or otherwise dispose
of (or solicit any offers to buy, purchase or otherwise acquire or take a pledge
of) any of the Shares except in compliance with the Securities Act, applicable
state securities laws and the respective rules and regulations promulgated
thereunder; and (iv) the Investor has, in connection with its decision to
purchase the number of Shares set forth on SCHEDULE I hereto, relied only upon
the representations and warranties of the Company contained herein. Subject to
Article IV herein, the Investor understands that its acquisition of the Shares
has not been registered under the Securities Act or registered or qualified
under any state securities law in reliance on specific exemptions therefrom,
which exemptions may depend upon, among other things, the bona fide nature of
the Investor's investment intent as expressed herein.

         SECTION 3.2 COMPLIANCE WITH SECURITIES LAWS. The Investor hereby
covenants with the Company not to make any sale of the Shares without complying
with the provisions of this Agreement, and without effectively causing the
prospectus delivery requirement under the Securities Act to be satisfied, and
the Investor acknowledges that the certificates evidencing the Shares will be
imprinted with a legend that prohibits their transfer except in accordance
therewith.

         SECTION 3.3 AUTHORITY OF INVESTOR; VALIDITY OF THIS AGREEMENT. The
Investor further represents and warrants to, and covenants with, the Company
that (i) the Investor has full right, power, authority and capacity to enter
into this Agreement and to consummate the transactions contemplated hereby and
has taken all necessary action to authorize the execution, delivery and
performance of this Agreement, and (ii) this Agreement constitutes a valid and
binding obligation of the Investor enforceable against the Investor in
accordance with its terms, except as enforceability may be limited by applicable
bankruptcy, insolvency, reorganization, moratorium or similar laws affecting
creditors' and contracting parties' rights generally and except as
enforceability may be subject to general principles of equity (regardless of
whether such enforceability is considered in a proceeding in equity or at law)
and except as the indemnification agreements of the Investor herein may be
legally unenforceable.

         SECTION 3.4 NO DISPOSITIONS. Investor will not, prior to the
effectiveness of the Registration Statement, sell, offer to sell, solicit offers
to buy, dispose of, loan, pledge or grant any right with respect to
(collectively, a "Disposition"), the Common Stock of the Company, nor will
Investor engage in any hedging or other transaction which is designed to or
could reasonably be expected to lead to or result in a Disposition of Common
Stock of the Company by the Investor or any other person or entity. Such
prohibited hedging or other transactions would include without limitation
effecting any short sale or having in effect any short position (whether or not
such sale or position is against the box and regardless of when such position
was entered into) or any purchase, sale or grant of any right (including without
limitation any put or call option) with respect to the Common Stock of the
Company or with respect to any security (other than a broad-based market basket
or index) that includes, relates to or derives any significant part of its value
from the Common Stock of the Company.


                                      -6-
<PAGE>

         SECTION 3.5 INVESTMENT DECISION BY INVESTOR. The Investor understands
that nothing in this Agreement or any other materials presented to the Investor
in connection with the purchase and sale of the Shares constitutes legal, tax or
investment advice. The Investor has consulted such legal, tax and investment
advisors as it, in its sole discretion, has deemed necessary or appropriate in
connection with its purchase of Shares.

         SECTION 3.6 FOREIGN SALES. The Investor acknowledges, represents and
agrees that no action has been or will be taken in any jurisdiction outside the
United States by the Company that would permit an offering of the Shares, or
possession or distribution of offering materials in connection with the issue of
the Shares, in any jurisdiction outside the United States where action for that
purpose is required. Each Investor outside the United States will comply with
all applicable laws and regulations in each foreign jurisdiction in which it
purchases, offers, sells or delivers Shares or has in its possession or
distributes any offering material, in all cases at its own expense.

                    ARTICLE IV -- REGISTRATION OF THE SHARES;
                       COMPLIANCE WITH THE SECURITIES ACT

          SECTION 4.1 REGISTRATION PROCEDURES AND EXPENSES. The Company shall:

                     (a) subject to receipt of necessary information from the 
Investors, prepare and file with the SEC, within twenty (20) business days after
the date hereof, the Registration Statement to enable the resale of the Shares
by the Investors from time to time through the automated quotation system of the
Nasdaq National Market (or such other exchange or trading market on which the
Shares are publicly traded, if applicable) or in privately-negotiated
transactions;

                      (b) use its best efforts, subject to receipt of necessary 
information from the Investors, to cause the Registration Statement to become
effective within 90 days after the Registration Statement is filed by the
Company;

                      (c) prepare and file promptly with the SEC such amendments
and supplements to the Registration Statement and the Prospectus used in
connection therewith (a "Prospectus") as may be necessary to keep the
Registration Statement current and effective for a period not exceeding, with
respect to each Investor's Shares purchased hereunder, the earlier of (i) the
second anniversary of the Closing Date, (ii) the date on which such Investor may
sell all Shares then held by such Investor without restriction under Rule 144(k)
of the Securities Act, or (iii) such time as all Shares purchased by such
Investor hereunder have been sold pursuant to a registration statement.

                       (d) furnish to each Investor with respect to the Shares 
registered under the Registration Statement such number of copies of
Prospectuses and preliminary Prospectuses (a "Preliminary Prospectus") in
conformity with the requirements of the Securities Act and such other documents
as such Investor may reasonably request promptly after receiving such request,
in order to facilitate the public sale or other disposition of all or any of the
Shares by such


                                      -7-
<PAGE>

Investor, provided, however, that the obligation of the Company to deliver
copies of Prospectuses or Preliminary Prospectuses to such Investor shall be
subject to the receipt by the Company of reasonable assurances from the Investor
that such Investor will comply with the applicable provisions of the Securities
Act and of such other securities or blue sky laws as may be applicable in
connection with any use of such Prospectuses or Preliminary Prospectuses;

                        (e) timely file documents required of the Company for 
blue sky clearance in states specified in writing by each Investor, provided,
however, that the Company shall not be required to qualify to do business or
consent to service of process in any jurisdiction in which it is not now so
qualified or has not so consented;

                        (f) bear all expenses in connection with the procedures 
in paragraph (a) through (e) of this Section 4.1 and the registration of the
Shares pursuant to the Registration Statement; and

                        (g) advise each Investor, promptly after it shall
receive notice or obtain knowledge of the issuance of any stop order by the SEC
delaying or suspending the effectiveness of the Registration Statement or of the
initiation or threat of any proceeding for that purpose; and it will promptly
use its best efforts to prevent the issuance of any stop order or to obtain its
withdrawal at the earliest possible moment if such stop order should be issued.

         The Company understands that each Investor disclaims being an
underwriter, but if such Investor is deemed an underwriter by the SEC, the
Company shall not be relieved of any obligations it has hereunder, PROVIDED,
HOWEVER that if the Company receives notification from the SEC that such
Investor is deemed an underwriter, then the period by which the Company is
obligated to submit an acceleration request to the SEC shall be extended for a
reasonable period not to exceed the earlier of (i) the 90th day after such SEC
notification, or (ii) 120 days after the initial filing of the Registration
Statement with the SEC.

          SECTION 4.2 TRANSFER OF SHARES AFTER REGISTRATION; SUSPENSION.

                        (a) Each Investor agrees that it will not effect any 
disposition of the Shares or its right to purchase the Shares that would
constitute a sale within the meaning of the Securities Act except as
contemplated in the Registration Statement referred to in Section 4.1 and as
described below or under Rule 144 of the Securities Act, and that it will
promptly notify the Company of any changes in the information set forth in the
Registration Statement regarding the Investor or its plan of distribution of the
Shares.

                        (b) Except in the event that paragraph (c) below 
applies, the Company shall (i) if deemed necessary by the Company, promptly
prepare and file from time to time with the SEC a post-effective amendment to
the Registration Statement or a supplement to the related Prospectus or a
supplement or amendment to any document incorporated therein by reference or
file any other required document so that such Registration Statement will not
contain an untrue 


                                      -8-
<PAGE>


statement of a material fact or omit to state a material fact required to be
stated therein or necessary to make the statements therein not misleading, and
so that, as thereafter delivered to purchasers of the Shares being sold
thereunder, such Prospectus will not contain an 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 light of the circumstances under which they
were made, not misleading; (ii) provide each Investor copies of any documents
filed pursuant to Section 4.2(b)(i); and (iii) inform each Investor that the
Company has complied with its obligations in Section 4.2(b)(i) (or that, if the
Company has filed a post-effective amendment to the Registration Statement which
has not yet been declared effective, the Company will notify each Investor to
that effect, will use its best efforts to secure the effectiveness of such
post-effective amendment as promptly as possible and will promptly notify the
Investor pursuant to Section 4.2(b)(i) hereof when the amendment has become
effective).

                        (c) Subject to paragraph (d) below, in the event (i) of 
any request by the SEC or any other federal or state governmental authority
during the period of effectiveness of the Registration Statement for amendments
or supplements to a Registration Statement or related Prospectus or for
additional information; (ii) of the issuance by the SEC or any other federal or
state governmental authority of any stop order suspending the effectiveness of a
Registration Statement or the initiation of any proceedings for that purpose;
(iii) of the receipt by the Company of any notification with respect to the
suspension of the qualification or exemption from qualification of any of the
Shares for sale in any jurisdiction or the initiation or threatening of any
proceeding for such purpose; or (iv) of any event or circumstance which
necessitates the making of any changes in the Registration Statement or
Prospectus, or any document incorporated or deemed to be incorporated therein by
reference, so that, in the Registration Statement, it will not contain any
untrue statement of a material fact or any omission to state a material fact
required to be stated therein or necessary to make the statements therein not
misleading, and that in the case of the Prospectus, it will not contain any
untrue statement of a material fact or any omission 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; then
the Company shall deliver a certificate in writing to each Investor (the
"Suspension Notice") to the effect of the foregoing and, upon receipt of such
Suspension Notice, such Investor will refrain from selling any Shares pursuant
to the Registration Statement (a "Suspension") until the Investor's receipt of
copies of a supplemented or amended Prospectus prepared and filed by the
Company, or until it is advised in writing by the Company that the current
Prospectus may be used, and has received copies of any additional or
supplemental filings that are incorporated or deemed incorporated by reference
in any such Prospectus. In the event of any Suspension, the Company will use its
best efforts to cause the use of the Prospectus so suspended to be resumed as
soon as reasonably practicable within ten (10) business days after the delivery
of a Suspension Notice to each Investor. In addition to and without limiting any
other remedies (including, without limitation, at law or at equity), available
to each Investor, such Investor shall be entitled to specific performance in the
event that the Company fails to comply with the provisions of this Section
4.2(c).

                        (d) Notwithstanding the foregoing paragraphs of this 
Section 4.2, each Investor shall not be prohibited from selling Shares under the
Registration Statement as a result of Suspensions on more than two (2) occasions
of not more than fifteen (15) days each in any twelve month period.


                                      -9-
<PAGE>


                        (e) Provided that a Suspension is not then in effect, 
each Investor may sell Shares under the Registration Statement, provided that
such Investor arranges for delivery of a current Prospectus to the transferee of
such Shares. Upon receipt of a request therefor, the Company has agreed promptly
to provide an adequate number of current Prospectuses to each Investor and to
supply copies to any other parties requiring such Prospectuses.

         SECTION 4.3 INDEMNIFICATION.  For the purpose of this Section 4.3 only:

                  (i) the term "Selling Stockholder" shall include each Investor
and any affiliate of such Investor;

                  (ii) the term "Registration Statement" shall include any final
Prospectus, exhibit, supplement or amendment included in or relating to the
Registration Statement referred to in Section 4.1; and

                  (iii) the term "untrue statement" shall include any untrue
statement or alleged untrue statement, or any omission or alleged omission to
state in the Registration Statement 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.

                        (a) The Company agrees to indemnify and hold harmless 
each Selling Stockholder from and against any losses, claims, damages or
liabilities to which such Selling Stockholder may become subject (under the
Securities Act or otherwise) insofar as such losses, claims, damages or
liabilities (or actions or proceedings in respect thereof) arise out of, or are
based upon (i) any untrue statement of a material fact contained in the
Registration Statement, or (ii) any failure by the Company to fulfill any
undertaking included in the Registration Statement, and the Company will
reimburse such Selling Stockholder for any reasonable legal or other expenses
reasonably incurred in investigating, defending or preparing to defend any such
action, proceeding or claim, or preparing to defend any such action, proceeding
or claim, PROVIDED, HOWEVER, that the Company shall not be liable in any such
case to the extent that such loss, claim, damage or liability arises out of, or
is based upon, an untrue statement made in such Registration Statement in
reliance upon and in conformity with written information furnished to the
Company by or on behalf of such Selling Stockholder specifically for use in
preparation of the Registration Statement or the failure of such Selling
Stockholder to comply with its covenants and agreements contained in Sections
3.2 or 4.2 hereof respecting sale of the Shares or any statement or omission in
any Prospectus that is corrected in any subsequent Prospectus that was delivered
to an Investor prior to the pertinent sale or sales by such Investor.

                           (b) Each Investor agrees to indemnify and hold 
harmless the Company (and each person, if any, who controls the Company within
the meaning of Section 15 of the Securities Act, each officer of the Company who
signs the Registration Statement and each director of the Company) from and
against any losses, claims, damages or liabilities to which the Company (or any
such officer, director or controlling person) may become subject (under the
Securities Act or otherwise), insofar as such losses, claims, damages or
liabilities (or actions or proceedings in respect thereof) arise out of, or are
based upon, (i) any failure to comply with the covenants and agreements
contained in Section 3.2 or 4.2 hereof respecting sale of the Shares, or 


                                      -10-
<PAGE>


(ii) any untrue statement of a material fact contained in the Registration
Statement if such untrue statement was made in reliance upon and in conformity
with written information furnished by or on behalf of such Investor specifically
for use in preparation of the Registration Statement, and such Investor will
reimburse the Company (or such officer, director or controlling person), as the
case may be, for any legal or other expenses reasonably incurred in
investigating, defending or preparing to defend any such action, proceeding or
claim.

                        (c) Promptly after receipt by any indemnified person of 
a notice of a claim or the beginning of any action in respect of which indemnity
is to be sought against an indemnifying person pursuant to this Section 4.3,
such indemnified person shall notify the indemnifying person in writing of such
claim or of the commencement of such action, but the omission to so notify the
indemnifying party will not relieve it from any liability which it may have to
any indemnified party under this Section 4.3 (except to the extent that such
omission materially and adversely affects the indemnifying party's ability to
defend such action) or from any liability otherwise than under this Section 4.3.
Subject to the provisions hereinafter stated, in case any such action shall be
brought against an indemnified person, the indemnifying person shall be entitled
to participate therein, and, to the extent that it shall elect by written notice
delivered to the indemnified party promptly after receiving the aforesaid notice
from such indemnified party, shall be entitled to assume the defense thereof,
with counsel reasonably satisfactory to such indemnified person. After notice
from the indemnifying person to such indemnified person of its election to
assume the defense thereof, such indemnifying person shall not be liable to such
indemnified person for any legal expenses subsequently incurred by such
indemnified person in connection with the defense thereof, PROVIDED, HOWEVER,
that if there exists or shall exist a conflict of interest that would make it
inappropriate, in the opinion of counsel to the indemnified person, for the same
counsel to represent both the indemnified person and such indemnifying person or
any affiliate or associate thereof, the indemnified person shall be entitled to
retain its own counsel at the expense of such indemnifying person; provided,
however, that no indemnifying person shall be responsible for the fees and
expenses of more than one separate counsel (together with appropriate local
counsel) for all indemnified parties. In no event shall any indemnifying person
be liable in respect of any amounts paid in settlement of any action unless the
indemnifying person shall have approved the terms of such settlement; PROVIDED
that such consent shall not be unreasonably withheld. No indemnifying person
shall, without the prior written consent of the indemnified person, effect any
settlement of any pending or threatened proceeding in respect of which any
indemnified person is or could have been a party and indemnification could have
been sought hereunder by such indemnified person, unless such settlement
includes an unconditional release of such indemnified person from all liability
on claims that are the subject matter of such proceeding.

                        (d) If the indemnification provided for in this Section
4.3 is unavailable to or insufficient to hold harmless an indemnified party
under subsection (a) or (b) above in respect of any losses, claims, damages or
liabilities (or actions or proceedings in respect thereof) referred to therein,
then each indemnifying party shall contribute to the amount paid or payable by
such indemnified party as a result of such losses, claims, damages or
liabilities (or actions in respect thereof) in such proportion as is appropriate
to reflect the relative fault of the Company on the one hand and the Investors
on the other in connection with the statements or omissions or other matters
which resulted in such losses, claims, damages or liabilities (or 



                                      -11-
<PAGE>


actions in respect thereof), as well as any other relevant equitable
considerations. The relative fault shall be determined by reference to, among
other things, in the case of an untrue statement, whether the untrue statement
relates to information supplied by the Company on the one hand or an Investor on
the other and the parties' relative intent, knowledge, access to information and
opportunity to correct or prevent such untrue statement. The Company and the
Investors agree that it would not be just and equitable if contribution pursuant
to this subsection (d) were determined by pro rata allocation (even if the
Investors were treated as one entity for such purpose) or by any other method of
allocation which does not take into account the equitable considerations
referred to above in this subsection (d). The amount paid or payable by an
indemnified party as a result of the losses, claims, damages or liabilities (or
actions in respect thereof) referred to above in this subsection (d) shall be
deemed to include any legal or other expenses reasonably incurred by such
indemnified party in connection with investigating or defending any such action
or claim. Notwithstanding the provisions of this subsection (d), no Investor
shall be required to contribute any amount in excess of the amount by which the
net amount received by such Investor from the sale of the Shares to which such
loss relates exceeds the amount of any damages which such Investor has otherwise
been required to pay by reason of such untrue statement. 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.

                           (e) The parties to this Agreement hereby acknowledge
that they are sophisticated business persons who were represented by counsel
during the negotiations regarding the provisions hereof including, without
limitation, the provisions of this Section 4.3, and are fully informed regarding
said provisions. They further acknowledge that the provisions of this Section
4.3 fairly allocate the risks in light of the ability of the parties to
investigate the Company and its business in order to assure that adequate
disclosure is made in the Registration Statement as required by the Securities
Act and the Exchange Act. The parties are advised that federal or state public
policy as interpreted by the courts in certain jurisdictions, may be contrary to
certain of the provisions of this Section 4.3, and the parties hereto hereby
expressly waive and relinquish any right or ability to assert such public policy
as a defense to a claim under this Section 4.3 and further agree not to attempt
to assert any such defense.

         SECTION 4.4 TERMINATION OF CONDITIONS AND OBLIGATIONS. The conditions
precedent imposed by Section 3 or this Section 4 upon the transferability of the
Shares shall cease and terminate as to any particular number of the Shares when
such Shares shall have been sold or otherwise disposed of in accordance with the
intended method of disposition set forth in the Registration Statement covering
such Shares or at such time as an opinion of counsel satisfactory to the Company
shall have been rendered to the effect that such conditions are not necessary in
order to comply with the Securities Act.

         SECTION 4.5 INFORMATION AVAILABLE. So long as the Registration
Statement is effective covering the resale of Shares owned by an Investor and
such Investor holds Shares subject to the Registration Statement, the Company
will furnish to such Investor:

                           (a) as soon as practicable after publicly available, 
one copy of (i) its Annual Reports to Stockholders (which Annual Reports shall
contain financial statements 


                                      -12-
<PAGE>


audited in accordance with generally accepted accounting principles by a
national firm of certified public accountants), (ii) if not included in
substance in the Annual Reports to Stockholders, its Annual Reports on Form
10-K, (iii) its Quarterly Reports on Form 10-Q, and (iv) a full copy of the
particular Registration Statement covering the Shares (the foregoing, in each
case, excluding exhibits); PROVIDED, HOWEVER, that in the event that the Company
is no longer required under the Exchange Act to file quarterly or annual reports
with the SEC, and for such time as the Company is not required to file such
reports (the "Non-Reporting Period"), the Company hereby agrees that it shall
use its best efforts to provide each Investor holding at least 70% of the Shares
purchased by such Investor hereunder (as adjusted for stock-splits, combinations
or other similar events) with audited financial statements within 120 days after
the end of the respective fiscal year following commencement of the
Non-Reporting Period and unaudited financial statements of the Company for the
applicable quarterly period of the Company within 45 days after the end of such
fiscal period, following commencement of the Non-Reporting Period.

                        (b) upon the reasonable written request of such 
Investor, all exhibits excluded by the parenthetical to subparagraph (a)(iv) of
this Section 4.5 as filed with the SEC and all other information that is made
available to stockholders at no cost to such Investor; and

                        (c) upon the reasonable written request of such 
Investor, an adequate number of copies of the Prospectuses to supply to any
other party requiring such Prospectuses; and the Company, upon the reasonable
request of such Investor, will meet with such Investor or a representative
thereof at the Company's headquarters to discuss all information relevant for
disclosure in the Registration Statement covering the Shares and will otherwise
cooperate with any Investor conducting an investigation for the purpose of
reducing or eliminating such Investor's exposure to liability under the
Securities Act, including the reasonable production of information at the
Company's headquarters; provided, that the Company shall not be required to
disclose any confidential information to or meet at its headquarters with any
Investor until and unless such Investor shall have entered into a
confidentiality agreement in form and substance reasonably satisfactory to the
Company with the Company with respect thereto.

          SECTION 4.6 "LOCK-UP" AGREEMENT. If the Company proposes to offer for
sale to the public any of its equity securities, and (i) if an Investor is an
"affiliate" of the Company or otherwise holds beneficially or of record ten
percent (10%) or more of the outstanding equity securities of the Company, and
(ii) if requested by the Company and an underwriter of shares of Common Stock or
other securities of the Company and (iii) if all other "affiliates" and 10%
stockholders that purchased securities directly from the Company after the date
hereof pursuant to a private placement of securities have signed or are
contractually obligated to sign a lock-up agreement (as described below), then
such Investor shall not offer, sell, grant any option or right to buy or sell,
or otherwise transfer or dispose of in any manner any Common Stock or other
securities of the Company held by it during the 90-day or such shorter period
following the effective date of the registration statement of the Company filed
under the Securities Act and will sign a "lock-up agreement" to such effect.
Such agreement shall be in writing and in form and substance reasonably
satisfactory to the Company and such underwriter and pursuant to customary and
prevailing terms and conditions. The Company may impose stop-transfer


                                      -13-
<PAGE>


instructions with respect to the securities subject to the foregoing
restrictions until the end of such 90-day period.

         SECTION 4.7 NASDAQ LISTING. The Company shall use best efforts to
comply in all material respects with the requirements of the National
Association of Securities Dealers, Inc. with respect to the issuance of the
Shares and the listing thereof on the Nasdaq National Market.

                    ARTICLE V -- SURVIVAL AND INDEMNIFICATION

         SECTION 5.1 SURVIVAL. Notwithstanding any examination made by or on
behalf of any party hereto, the knowledge of any party or the acceptance by any
party of any certificate or opinion, each representation and warranty contained
herein shall survive the Closing and shall be fully effective and enforceable
for one year after the Closing, and each covenant contained herein shall survive
the Closing and shall be fully effective and enforceable.

         SECTION 5.2 INDEMNIFICATION.

                        (a) The Company shall indemnify each Investor, its
shareholders, officers, directors, employees, agents and representatives against
any damages, claims, losses, liabilities and expenses (including reasonable
counsel fees and expenses) which may be suffered or incurred by any of them as a
result of a breach of any representation, warranty or covenant made by the
Company in this Agreement;

                        (b) Each Investor agrees to indemnify the Company and 
its shareholders, officers, directors, employees, agents and representatives
against any damages, claims, losses, liabilities and expenses (including
reasonable counsel fees and other expenses) which may be suffered or incurred by
it as a result of any breach of any representation, warranty, or covenant made
by such Investor in this Agreement; and

                  (c) In case any proceeding (including any governmental
investigation) shall be instituted involving any person in respect of which
indemnity may be sought pursuant to this Section, such person (the "indemnified
party") shall promptly notify the person against whom such indemnity may be
sought (the "indemnifying party") in writing of the occurrence of the facts and
circumstances giving rise to such claim. The failure of any person to deliver
the notice required by this Section 5.2(c) shall not in any way affect the
indemnifying party's indemnification obligations hereunder except and only to
the extent tat the indemnifying party is actually prejudiced thereby. In case
any such proceeding shall be brought against any indemnified party and it shall
notify the indemnifying party of the commencement thereof, the indemnifying
party shall be entitled to participate therein and, to the extent that it shall
which, jointly with any other indemnifying party similarly notified, to assume
the defense thereof, with counsel satisfactory to such indemnified party and
shall pay as incurred the fees and expenses of such counsel related to such
proceeding. In any such proceeding, any indemnified party shall have 
the right to retain its own counsel or pay its own expenses. Notwithstanding the
foregoing, the indemnifying party shall pay as incurred the fees and expenses of
the counsel retained by the indemnified party in the event (i) the indemnifying
party and the indemnified party shall have 

                                      -14-
<PAGE>


mutually agreed to the retention of such counsel or (ii) the named parties to
any such proceedings (including any impleaded parties) include both the
indemnifying party and the indemnified party and representations of both parties
by the same counsel would be inappropriate due to actual or potential differing
interests between them. The indemnifying party shall not be liable for any
settlement of any proceeding effected without its written consent (which shall
not be unreasonably withheld) but if settled with such consent or if there be a
final judgment for the plaintiff, the indemnifying party agrees to indemnify the
indemnified party from and against any loss or liability by reason of such
settlement or judgment.

                           ARTICLE VI -- MISCELLANEOUS

         SECTION 6.1 NOTICES. All notices, requests, consents and other
communications hereunder shall be in writing, shall be addressed to the
receiving party's address set forth below or to such other address as a party
may designated by notice hereunder, and shall be either (i) delivered by hand,
(ii) made by telex, telecopy or facsimile transmission, (iii) sent by overnight
courier, or (iv) sent by registered mail, return receipt requested, postage
prepaid.

         If to an Investor:  The address specified for such Investor on 
                             SCHEDULE I.

         With a copy to:     Paramount Capital
                             787 Seventh Street
                             New York, NY  10019
                             Attention:  David Tanen, Esq.
                             Facsimile:  212-554-4498

         If to the Company:  Repligen Corporation
                             117 Fourth Avenue
                             Needham, MA  02194
                             Attention:  Chief Executive Officer
                             Facsimile:  781-453-0048

         With a copy to:     Testa, Hurwitz & Thibeault, LLP
                             125 High Street
                             Boston, MA  02110
                             Attention:  Lawrence S. Wittenberg, Esq.
                             Facsimile:  617-248-7100


All notices, requests, consents and other communications hereunder shall be
deemed to have been given together (i) if by hand, at the time of the delivery
thereof to the receiving party at the address of such party set forth above;
(ii) if by telex, telecopy or facsimile transmission, one (1) day after the time
that receipt thereof has been acknowledged by electronic confirmation or
otherwise; (iii) if sent by overnight courier, on the next business day
following the day such notice is delivered to the courier service; or (iv) if
sent by registered mail, on the 5th business day following the day such mailing
is made.


                                      -15-
<PAGE>


         SECTION 6.2 ENTIRE AGREEMENT. This Agreement, including exhibits, or
other documents referred to herein or that specifically indicate that they were
delivered to the Investors in connection with this Agreement, embodies the
entire agreement and understanding between the parties hereto with respect to
the subject matter hereof and supersedes all prior oral or written agreements
and understandings relating to the subject matter hereof. No statement,
representation, warranty, covenant or agreement of any kind not expressly set
forth in this Agreement or in any document that specifically indicates that it
was delivered to the Investors in connection with this Agreement shall affect,
or be used to interpret, change or restrict, the express terms and provisions of
this Agreement.

         SECTION 6.3 AMENDMENTS. The terms and provisions of the Agreement may
be modified, amended or waived, or consent for the departure therefrom granted,
only by written consent of the Company and the Investors holding at least 50% of
the Shares then held by all Investors. No such waiver or consent shall be deemed
to be an Agreement, whether or not similar. Each such waiver or consent shall be
effective only in the specific instance and for the purpose for which it was
given, and shall not constitute a continuing waiver or consent.

         SECTION 6.4 ASSIGNMENT. The rights and obligations under this Agreement
may not be assigned by any party hereto without the prior written consent of the
Company and Investors holding in the aggregate at least 50% of the Shares then
held by all Investors, which consent shall not be unreasonably withheld, except
that an Investor may transfer its rights and obligations to an "affiliate" (as
such term is defined under Rule 144 of the Securities Act) of such Investor.

         SECTION 6.5 BENEFIT. All statements, representations, warranties,
covenants and agreements in this Agreement shall be binding on the parties
hereto and shall inure to the benefit of the respective successors and permitted
assigns of each party hereto. Nothing in this Agreement shall be construed to
create any rights or obligations except among the parties hereto, and no person
or entity shall be regarded as a third-party beneficiary of this Agreement.

         SECTION 6.6 GOVERNING LAW. This Agreement and the rights and
obligations of the partied hereunder shall be construed in accordance with and
governed by the law of the Commonwealth of Massachusetts, without giving effect
to the conflict of law principles thereof.

         SECTION 6.7 SEVERABILITY. In the event that any court of competent
jurisdiction shall determine that any provision, or any portion thereof,
contained in this Agreement shall be unreasonable or unenforceable in any
respect, then such provision shall be deemed limited to the extent that such
court deems it reasonable and enforceable, and as so limited shall remain in
full force and effect. In the event that such court shall deem any such
provision, or portion thereof, wholly unenforceable, the remaining provisions of
this Agreement shall nevertheless remain in full force and effect.

         SECTION 6.8 HEADINGS AND CAPTIONS. The headings and captions of the
various subdivisions of this Agreement are for convenience of reference only and
shall in no way modify, or affect the meaning or constructions of any of the
terms or provisions hereof.



                                      -16-
<PAGE>


         SECTION 6.9 NO WAIVER OF RIGHTS, POWERS AND REMEDIES. No failure or
delay by a party hereto in exercising any right, power or remedy under this
Agreement, and no course of dealing between the parties hereto, shall operate as
a waiver of any such right, power or remedy of the party. No single or partial
exercise of any right, power or remedy under this Agreement by a party hereto,
nor any abandonment or discontinuance of steps to enforce any such right, power
or remedy, shall preclude such party from other or further exercise thereof or
the exercise of any other right, power or remedy hereunder. The election of any
remedy by a party hereto shall not constitute a waiver of the right of such
party to pursue other available remedies. No notice to or demand on a party not
expressly required under this Agreement shall entitle the party receiving such
notice or demand to any other or further notice or demand in similar or other
circumstances or constitute a waiver of the rights of the party giving such
notice or demand to any other or further action in any circumstances without
such notice or demand.

         SECTION 6.10 EXPENSES. Except as provided in Section 4.1(f), Section
4.3, Section 4.5(b) or Section 5.2, each of the parties shall pay its own fees
and expenses (including the fees of any attorneys, accountants, appraisers or
others engaged by such party) in connection with this Agreement and the
transactions contemplated hereby whether or not the transactions contemplated
hereby are consummated.

         SECTION 6.11 BROKERS. Each of the parties hereto represents and
warrants to the other that no broker, finder or other financial consultant has
acted on its behalf in connection with this Agreement or the transactions
contemplated hereby in such a way as to create any liability on the other. Each
of the parties hereto agrees to indemnify and save the other harmless from any
claim or demand for commission, or for other compensation by any broker, finder,
financial consultant or similar agent claiming to have been employed by or on
behalf of such party and to bear the cost of legal expenses incurred in
defending against any such claim.

         SECTION 6.12 CONFIDENTIALITY. Each Investor acknowledges and agrees
that any information or data it has acquired from the Company, which is clearly
designated in writing as confidential and is not otherwise properly in the
public domain, was received in confidence. Each Investor agrees not to divulge,
communicate or disclose, except as may be required by law or for the performance
of this Agreement, or use to the detriment of the Company or for the benefit of
any other person or persons, or misuse in any way, any confidential information
of the Company.

         SECTION 6.13 COUNTERPARTS. This Agreement may be executed in one or
more counterparts, and by different parties hereto on separate counterparts,
each of which shall be deemed an original, but all of which together shall
constitute one and the same instrument.

         SECTION 6.14 FURTHER ASSISTANCE. In case at any time after the Closing
any further action is necessary or desirable to carry out the purposes of this
Agreement, the Company and each Investor will take such further action as the
other party may reasonably request, all at the sole cost and expense of the
requesting party (unless the requesting party is entitled to indemnification
under Article IV or V).



                                      -17-
<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 13th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer


                                    J. F. Shea Co., Inc. as Nominee 1999-28
                                    -------------------------------------------

                                    For 200,000 Shares
                                    -------------------------------------------


                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Edmund H. Shea, Jr.
                                        ---------------------------------------
                                        Name:   Edmund H. Shea, Jr.
                                        Title:  Vice President



                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

                                    Keys Foundation, Curacao
                                    -------------------------------------------

                                    p.a. Dr. Tis Prager
                                    -------------------------------------------

                                    
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Tis Prager, Chairman
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        Signed on May 14, 1999
                                        for the purchase of 200,000 shares



                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

                                    Tis Prager, Zumikon
                                    -------------------------------------------

                                    p.a. Dr. Tis Prager
                                    -------------------------------------------


                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Tis Prager
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        Signed on May 14, 1999
                                        for the purchase of 20,000 shares



                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

                                    Bruno Widmer, Zurich
                                    -------------------------------------------

                                    p.a. Dr. Tis Prager
                                    -------------------------------------------


                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Tis Prager, attorney in fact
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        Signed on May 14, 1999
                                        for the purchase of 10,000 shares



                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

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

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

                                    Elliott Broidy
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Elliott Broidy
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        # Shares: 40,000
                                        ---------------------------------------




                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

                                    Peter L. Jensen
                                    -------------------------------------------

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

                                    
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Peter L. Jensen
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        10,000 Shares




                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

                                    Scott A. Katzmann
                                    -------------------------------------------

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

                                    
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Scott A. Katzmann
                                        ---------------------------------------
                                        Name:   Scott A. Katzmann
                                        Title:  

                                        # Shares: 20,000
                                        ---------------------------------------




                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

                                    M.S.B. Research
                                    -------------------------------------------

                                    P.O. Box 223
                                    -------------------------------------------

                                    Cold Spring Harbor NY 11749
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Marc Berg
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        # Shares: 100,000
                                        ---------------------------------------




                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

                                    Esther Berg
                                    -------------------------------------------

                                    67-25 161St.
                                    -------------------------------------------

                                    Flushing, NY 11365
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Esther Berg
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        # Shares: 100,000
                                        ---------------------------------------




                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

                                    Timothy McInerney
                                    -------------------------------------------

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

                                    
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Timonthy McInerney
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        # Shares: 20,000
                                        ---------------------------------------




                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

                                    Martin P. Sutter
                                    -------------------------------------------

                                    4026 Windelorst Dr.
                                    -------------------------------------------

                                    Montgomery, TX 77356
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Martin P. Sutter
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        # Shares: 25,000
                                        ---------------------------------------



                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

                                    E. Justin Kelly
                                    -------------------------------------------

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

                                    
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Justin Kelly
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        # Shares: 10,000
                                        ---------------------------------------



                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

                                    Joseph Edelman
                                    -------------------------------------------

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

                                    
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Joseph Edelman
                                        ---------------------------------------
                                        Name:   
                                        Title:  Portfolio Manager

                                        # Shares: 20,000
                                        ---------------------------------------



                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

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

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

                                    Joseph Natiello
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Joseph Natiello
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        # Shares: 20,000
                                        ---------------------------------------



                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

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

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

                                    Ewa Lipton
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Ewa Lipton
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        # Shares: 20,000
                                        ---------------------------------------



                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                By: /s/ Walter C. Herlihy
                                    ---------------------------------------
                                    Name:   Walter C. Herlihy
                                    Title:  Chief Executive Officer

                                Joseph Strassman and Tenants in Common
                                -------------------------------------------

                                Barbara Strassman
                                -------------------------------------------

                                
                                -------------------------------------------
                                (Print or Type Name of Investor)

                                By: /s/ Joseph Strassman and Barbara Strassman
                                    ------------------------------------------
                                    Name:   
                                    Title:  

                                    # Shares: 40,000
                                    ---------------------------------------



                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

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

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

                                    
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Mark Mazzer
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        # Shares: 10,000
                                        ---------------------------------------



                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

                                    Frederick J. Korniewicz
                                    -------------------------------------------

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

                                    
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Frederick Korniewicz
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        # Shares: 15,000
                                        ---------------------------------------



                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

                                    Mark Walko
                                    -------------------------------------------

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

                                    
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Mark Walko
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        # Shares: 10,000
                                        ---------------------------------------



                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

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

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

                                    Fred H. Mermelstein
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Fred Mermelstein
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        # Shares: 4,000
                                        ---------------------------------------



                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

                                    Aries Domestic Fund 15,000
                                    -------------------------------------------

                                    Aries Master Fund 35,000
                                    -------------------------------------------

                                    
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Lindsey Rosenwald
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        # Shares: 50,000
                                        ---------------------------------------



                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

                                    Domaco Venture Capital Fund
                                    -------------------------------------------

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

                                    
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Domaco Venture Capital Fund 
                                            Jack Pollack
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        # Shares: 10,000
                                        ---------------------------------------



                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

                                    Anthony G. Polak "S"
                                    -------------------------------------------

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

                                    
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Anthony G. Polak "S"
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        # Shares: 10,000
                                        ---------------------------------------



                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

                                    Anthony G. Polak
                                    -------------------------------------------

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

                                    
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Anthony G. Polak
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        # Shares: 10,000
                                        ---------------------------------------



                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

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

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

                                    Jonathan Rothschild
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Jonathan Rothschild
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        # Shares: 10,000
                                        ---------------------------------------



                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

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

                                    Arterio, Inc.
                                    -------------------------------------------

                                    Jonathan Rothschild, President
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Jonathan Rothschild, President
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        # Shares: 10,000
                                        ---------------------------------------



                                      -18-

<PAGE>



         IN WITNESS WHEREOF, the undersigned have executed this Stock Purchase
Agreement this 14th day of May, 1999.

                                    REPLIGEN CORPORATION


                                    By: /s/ Walter C. Herlihy
                                        ---------------------------------------
                                        Name:   Walter C. Herlihy
                                        Title:  Chief Executive Officer

                                    Ronald Lazar
                                    -------------------------------------------

                                    3 Horizon Rd.
                                    -------------------------------------------

                                    Fort Lee, NJ 07024
                                    -------------------------------------------
                                    (Print or Type Name of Investor)

                                    By: /s/ Ronald Lazar
                                        ---------------------------------------
                                        Name:   
                                        Title:  

                                        # Shares: 6,000
                                        ---------------------------------------



                                      -18-


<PAGE>


                                                                    Exhibit 99.1


REPLIGEN ANNOUNCES $9 MILLION PRIVATE PLACEMENT

ADDITIONAL CAPITAL TO SUPPORT CLINICAL TRIALS OF SECRETIN IN AUTISM
REPLIGEN ANNOUNCES $9 MILLION PRIVATE PLACEMENT

NEEDHAM, MA -- May 17, 1999 -- Repligen Corporation (Nasdaq: RGEN) announced
today that it has entered into definitive agreements to raise $9 million through
a private placement of Repligen's common stock. The Company will issue 3.6
million shares at $2.50 per share.

         The primary use of the proceeds of this financing will be to fund
clinical trials of secretin in autism. Anecdotal reports have indicated that
secretin may produce improvements in gastrointestinal function, language, social
behavior and sleep in autistic children. In March 1999, Repligen acquired the
rights to patent applications for the use of secretin in autism. The Company
plans to manufacture a synthetic, human form of secretin and initiate a
controlled clinical trial by years' end.

         "This financing will increase our cash position to approximately $12
million," stated Walter C. Herlihy, President and CEO of Repligen. "We expect
that this capital will enable us to aggressively pursue the clinical development
of human secretin for autism for at least two years without the need for
additional financing."

         Participants in the financing included funds managed by Wellington
Management Company, LLP. Paramount Capital acted as a financial advisor to the
Company. The financing will close upon the effectiveness of a registration
statement which the Company plans to file with the SEC within 20 days.

         Repligen Corporation develops new drugs for autism, organ transplant
and cancer. The Company also manufactures and markets a set of patented products
based on Protein A which are used by the pharmaceutical industry to produce
therapeutic antibodies. Its corporate headquarters are located at 117 Fourth
Avenue, Needham, MA 02494. Additional information may be requested from
www.repligen.com. 


         This press release contains forward-looking statements based on 
current management expectations. There are certain key factors which could 
cause future results to differ materially from those anticipated by 
management. Such factors include, but are not limited to: uncertainty in the 
realization of future revenues, the uncertain timeline for clinical activity, 
results of pending or future clinical trials, the Company's ability to 
continue to establish collaborative arrangements with third parties; the 
Company's ability to maintain financial stability; the technical risks 
associated with development and manufacture of products; the fact that there 
can be no assurances that patents relating to the Company's potential 
products will afford adequate protection to the Company, the risks of 
technological change and competition, and the competitive environment of the 
biotechnology and pharmaceutical industries. These factors are more fully 
discussed in the Company's periodic filings with the Securities and Exchange 
Commission.


                                       ####



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