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                                                                    Exhibit 4.2

                              REPLIGEN CORPORATION

               WARRANT FOR THE PURCHASE OF SHARES OF COMMON STOCK


Name and address of
Registered Holder:                            John E. Abdo TTEE U/A DTD 3/15/76
                                              FBO John E. Abdo
                                              1350 NE 56th
                                              Ft. Lauderdale, FL 33334

No.                                           2,900 Shares
   ----------------------



         IN CONSIDERATION OF the Release entered into by and among Repligen
Corporation, a Delaware corporation (the "Company"), John E. Abdo and John E.
Abdo TTEE U/A DTD 3/15/76 FBO John E. Abdo (the "Initial Holder") dated as of
the date hereof and for value received, the Company hereby grants the rights
herein specified and certifies that the Initial Holder or any registered
assignee of the Initial Holder (each of the Initial Holder and any such
registered assignee being hereinafter referred to as the "Holder") is entitled,
subject to the conditions and upon the terms of this Warrant, to purchase from
the Company, at any time or from time to time during the Exercise Period (as
defined in Section 1 hereof), 2,900 shares of Common Stock (as defined in
Section 1 hereof). The number of shares of Common Stock to be received upon the
exercise of this Warrant and the Exercise Price are subject to adjustment from
time to time as hereinafter set forth.

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

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

         "Closing Price" means the closing price per share of the Common Stock
on the principal national securities exchange on which the Common Stock is
listed or admitted to trading or, if not listed or traded on any such exchange,
on the Nasdaq National Market, or if not listed or traded on any such exchange
or system, the average of the bid and asked price per share on Nasdaq National
Market or, if such quotations are not available, the fair market value as
reasonably determined by the Board of Directors of the Company or any committee
of such Board.

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

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


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         "Exercise Period" means the period beginning on April 14, 2000 and
ending on July 6, 2000.

         "Exercise Price" means $9.00 per share subject to change or adjustment
pursuant to Section 8 hereof.

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

         "National Market" means the National Market of Nasdaq.

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

         "Warrant" means this Warrant and any Warrant or Warrants which may be
issued pursuant to Section 5 hereof in substitution or exchange for or upon
transfer of this Warrant, any Warrant which may be issued pursuant to Section 2
hereof upon partial exercise of this Warrant and any Warrant which may be issued
pursuant to Section 6 hereof upon the loss, theft, destruction or mutilation of
this Warrant.

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

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

2.       EXERCISE OF WARRANT. This Warrant may be exercised, in whole or in
part, at any time or from time to time during the Exercise Period, by
presentation and surrender hereof to the Company at its principal office at the
address set forth on the signature page hereof (or at such other address of the
Company or any agent appointed by the Company to act hereunder as the


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

3.       RESERVATION OF SHARES. The Company agrees that at all times it will
keep reserved solely for issuance and delivery pursuant to the Warrants the
number of shares of its Common Stock that are or would be issuable from time to
time upon exercise of this Warrant. All such shares shall be duly authorized
and, when issued upon such exercise, shall be validly issued, fully paid and
nonassessable, free and clear of all liens, security interests, charges and
other encumbrances or restrictions on sale and free of all preemptive rights.
Before taking any action that would cause an adjustment pursuant to Section 8
hereof reducing the Exercise Price below the then par value (if any) of the
Warrant Shares issuable upon exercise of this Warrant, the Company will take any
corporate action that may, in the opinion of its counsel, be necessary in order
that the Company may validly and legally issue fully paid and nonassessable
Warrant Shares at the Exercise Price as so adjusted.

4.       WARRANT SHARE REGISTRATION RIGHTS.

         (a)  The Company will use commercially reasonable efforts to (i)
prepare and file with the Securities and Exchange Commission (the "SEC") under
the Act a registration statement with respect to the Warrant Shares issuable
upon exercise of this Warrant, (ii) cause such registration statement to become
effective under the Act within 45 days from the date hereof and remain effective
under the Act during the Exercise Period while any of such Warrants are
outstanding and (iii) register or qualify such Warrant Shares under the
securities or Blue Sky laws of each jurisdiction within the United States in
which such registration or qualification is necessary in connection with the
issuance and delivery of such Warrant Shares to the Holders of the Warrants.

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

              (i)       to use reasonable commercial efforts, to prepare and
         file with the SEC such amendments and supplements to such registration
         statement and the prospectus used


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         in connection therewith as may be necessary to keep such registration
         statement effective throughout the Exercise Period;

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

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

5.       EXCHANGE, TRANSFER OR ASSIGNMENT OF WARRANT.

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

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

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

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

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


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

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

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


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         subsection (ii) shall require that an adjustment in the number of
         shares receivable hereunder or the Exercise Price be made effective as
         of a record date for a specified event, the Company may elect to defer
         until the occurrence of such event issuing to the Holder of any Warrant
         exercised after such record date the number of Warrant Shares, if any,
         issuable upon such exercise over and above the number of Warrant
         Shares, if any, issuable upon such exercise on the basis of the
         Exercise Price in effect prior to such adjustment; PROVIDED, HOWEVER,
         that the Company shall deliver to such Holder a due bill or other
         appropriate instrument evidencing such Holder's right to receive such
         additional Warrant Shares upon the occurrence of the event requiring
         such adjustments.

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

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    receivable hereunder or the Exercise Price be made effective as of a
    record date for a specified event, the Company may elect to defer until
    the occurrence of such event issuing to the Holder of any Warrant
    exercised after such record date the number of Warrant Shares, if any,
    issuable upon such exercise over and above the number of Warrant Shares,
    if any, issuable upon such exercise on the basis of the Exercise Price in
    effect prior to such adjustment; PROVIDED, HOWEVER, that the Company
    shall deliver to such Holder a due bill or other appropriate instrument
    evidencing such Holder's right to receive such additional Warrant Shares
    upon the occurrence of the event requiring such adjustments.

         (b)  REORGANIZATION EVENT.

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

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

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

         (d)  CARRYOVER. Notwithstanding any other provision of this Section 8,
no adjustment shall be made to the number of shares of Common Stock (or other
securities deliverable hereunder) to be delivered to each Holder (or to the
Exercise Price) if such adjustment would represent less than one percent of the
number of shares to be so delivered, but any such


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adjustment shall be carried forward and shall be made at the time and together
with the next subsequent adjustment which, together with any adjustments so
carried forward, shall amount to one percent or more of the number of shares to
be so delivered.

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

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

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

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

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

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

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

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


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

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

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

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

13.      SUCCESSORS. All of the provisions of this Warrant by or for the benefit
of the Company or the Holder shall bind and inure to the benefit of their
respective successors and assigns.

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


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

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

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

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

         IN WITNESS WHEREOF, the Company has duly caused this Warrant to be
signed and attested by its duly authorized officers as of April 14, 2000.


                                       REPLIGEN CORPORATION

                                       By:/s/Walter C. Herlihy
                                          -----------------------
                                          Walter C. Herlihy
                                          President and CEO

                                       Address:
                                       117 Fourth Avenue
                                       Needham, MA 02194
                                       Attention: Secretary


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                                  PURCHASE FORM

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



Date:                 , 2000    --------------------------------------------
     -----------------                           [Signed]



                                --------------------------------------------
                                             [Street Address]


                                --------------------------------------------
                                             [City and State]


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                             WARRANT ASSIGNMENT FORM

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

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

Address:
        --------------------------------------------


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


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

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


Date:                 , 2000    --------------------------------------------
     -----------------                           [Signed]


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