<PAGE>

THIS WARRANT HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
AMENDED, OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT BE SOLD OR
TRANSFERRED UNLESS SUCH SALE OR TRANSFER IS IN ACCORDANCE WITH THE REGISTRATION
REQUIREMENTS OF SUCH ACT AND APPLICABLE LAWS OR SOME OTHER EXEMPTION FROM THE
REGISTRATION REQUIREMENTS OF SUCH ACT AND APPLICABLE LAWS IS AVAILABLE WITH
RESPECT THERETO.


                          COMMON STOCK PURCHASE WARRANT

Warrant No. __                                        Number of Shares: 129,946
                              REPLIGEN CORPORATION


                            Void after March 9, 2005



         1. ISSUANCE. This Warrant is issued to Paramount Capital, Inc.
("Paramount") on this 9th day of March, 2000 (the "Original Issue Date") by
RepliGen Corporation, a Delaware corporation (hereinafter with its successors
called the "Company"), pursuant to a certain Finders Agreement dated as of March
8, 2000 by and between Paramount and the Company.

         2. PURCHASE PRICE; NUMBER OF SHARES. Subject to the terms and
conditions hereinafter set forth, the registered holder of this Warrant (the
"Holder"), commencing on September 9, 2000, is entitled upon surrender of this
Warrant with the subscription form annexed hereto duly executed, at the office
of the Company, 117 Fourth Avenue, Needham, MA 02494, or such other office as
the Company shall notify the Holder of in writing, to purchase from the Company
at a price per share of $9.49 (the "Purchase Price"), one hundred twenty-nine
thousand nine hundred forty-six (129,946) fully paid and nonassessable shares of
Common Stock, $.01 par value per share, of the Company (the "Common Stock").
Until such time as this Warrant is exercised in full or expires, the Purchase
Price and the securities issuable upon exercise of this Warrant are subject to
adjustment as hereinafter provided.

         3. PAYMENT OF PURCHASE PRICE. The Purchase Price may be paid (i) in
cash or by check, (ii) by the surrender by the Holder to the Company of any
promissory notes or other obligations issued by the Company, with all such notes
and obligations so surrendered being credited against the Purchase Price in an
amount equal to the principal amount thereof plus accrued interest to the date
of surrender, (iii) through delivery by the Holder to the Company of other
securities issued by the Company, with such securities being credited against
the Purchase Price in an amount equal to the fair market value thereof, as
determined in good faith by the Board of Directors of the Company (the "Board"),
or (iv) by any combination of the foregoing. The Board shall promptly respond in
writing to an inquiry by the Holder as to the fair market

<PAGE>

                                      -2-

value of any securities  the Holder may wish to deliver to the Company  pursuant
to clause (iii) above.

         4. NET ISSUE ELECTION. The Holder may elect to receive, without the
payment by the Holder of any additional consideration, shares equal to the value
of this Warrant or any portion hereof by the surrender of this Warrant or such
portion to the Company, with the net issue election notice annexed hereto duly
executed, at the office of the Company. Thereupon, the Company shall issue to
the Holder such number of fully paid and nonassessable shares of Common Stock as
is computed using the following formula:

                                   X = Y (A-B)
                                       ------
                                        A

where

                           X = the number of shares to be issued to the Holder
pursuant to this Section 4.

                           Y = the number of shares covered by this Warrant in
                  respect of which the net issue election is made pursuant to
                  this Section 4.

                           A = the fair market value of one share of Common
                  Stock, which shall be equal to the average closing price on
                  the Nasdaq National Market of the Common Stock over the ten
                  (10) day period prior to the date the net issue election is
                  made pursuant to this Section 4.

                           B = the Purchase Price in effect under this Warrant
                  at the time the net issue election is made pursuant to this
                  Section 4.

The Board shall promptly respond in writing to an inquiry by the Holder as to
the fair market value of one share of Common Stock.

         5. PARTIAL EXERCISE. This Warrant may be exercised in part, and the
Holder shall be entitled to receive a new warrant, which shall be dated as of
the date of this Warrant, covering the number of shares in respect of which this
Warrant shall not have been exercised.

         6. ISSUANCE DATE. The person or persons in whose name or names any
certificate representing shares of Common Stock is issued hereunder shall be
deemed to have become the holder of record of the shares represented thereby as
at the close of business on the date this Warrant is exercised with respect to
such shares, whether or not the transfer books of the Company shall be closed.

         7. EXPIRATION DATE. This Warrant shall expire at the close of business
on March 9, 2005, and shall be void thereafter.

<PAGE>

                                      -3-

         8. RESERVED SHARES; VALID ISSUANCE. The Company covenants that it will
at all times from and after the date hereof reserve and keep available such
number of its authorized shares of Common Stock, free from all preemptive or
similar rights therein, as will be sufficient to permit the exercise of this
Warrant in full. The Company further covenants that such shares as may be issued
pursuant to the exercise of this Warrant will, upon issuance, be duly and
validly issued, fully paid and nonassessable and free from all taxes, liens and
charges with respect to the issuance thereof other than those caused or suffered
by the Holder hereof.

         9. DIVIDENDS. If after the Original Issue Date the Company shall
subdivide the Common Stock, by split-up or otherwise, or combine the Common
Stock, or issue additional shares of Common Stock in payment of a stock dividend
on the Common Stock, the number of shares issuable on the exercise of this
Warrant shall forthwith be proportionately increased in the case of a
subdivision or stock dividend, or proportionately decreased in the case of a
combination, and the Purchase Price shall forthwith be proportionately decreased
in the case of a subdivision or stock dividend, or proportionately increased in
the case of a combination.

         10. MERGERS AND RECLASSIFICATIONS. If after the Original Issue Date
there shall be any reclassification, capital reorganization or change of the
Common Stock (other than as a result of a subdivision, combination or stock
dividend provided for in Section 9 hereof), or any consolidation of the Company
with, or merger of the Company into, another corporation or other business
organization (other than a consolidation or merger in which the Company is the
continuing corporation and which does not result in any reclassification or
change of the outstanding Common Stock), or any sale or conveyance to another
corporation or other business organization of all or substantially all of the
assets of the Company, then, as a condition of such reclassification,
reorganization, change, consolidation, merger, sale or conveyance, lawful
provisions shall be made, and duly executed documents evidencing the same from
the Company or its successor shall be delivered to the Holder, so that the
Holder shall thereafter have the right to purchase, at a total price not to
exceed that payable upon the exercise of this Warrant in full, the kind and
amount of shares of stock and other securities and property receivable upon such
reclassification, reorganization, change, consolidation, merger, sale or
conveyance by a holder of the number of shares of Common Stock which might have
been purchased by the Holder immediately prior to such reclassification,
reorganization, change, consolidation, merger, sale or conveyance, and in any
such case appropriate provisions shall be made with respect to the rights and
interest of the Holder to the end that the provisions hereof (including without
limitation, provisions for the adjustment of the Purchase Price and the number
of shares issuable hereunder) shall thereafter be applicable in relation to any
shares of stock or other securities and property thereafter deliverable upon
exercise hereof.

         11. PURCHASE PRICE ADJUSTMENTS. If and whenever after the Original
Issue Date, but prior to March 9, 2001, the Company shall issue or sell any
shares of its Common Stock for a consideration per share less than the average
closing bid price for the five (5) days preceding any such issuance (the "Bid
Price"), and/or the Company shall issue or sell any securities convertible into
or exercisable for shares of its Common Stock for a consideration per share less
than the Bid Price on the date of such issue or sale, except in all instances
for the exercise of any options or warrants outstanding on the Original Issue
Date, or upon the grant or exercise of any options or

<PAGE>

                                      -4-

stock issued after the Original Issue Date pursuant to any stock option and
incentive plan approved by the Board of Directors of the Company, forthwith upon
such issue or sale, the number of shares which may be purchased upon exercise of
this Warrant shall be adjusted (rounded up to the nearest whole share) obtained
by DIVIDING the number of shares which may be acquired upon the exercise of this
Warrant BY a fraction, the NUMERATOR of which shall be the SUM of (i) the number
of shares of Common Stock outstanding immediately prior to such issue or sale
multiplied by the Bid Price PLUS (ii) the consideration received by the Company
upon such issue or sale, and the DENOMINATOR of which shall be the PRODUCT of
(x) the total number of shares of Common Stock outstanding immediately after
such issue or sale, MULTIPLIED by (y) the Bid Price.

         12. FRACTIONAL SHARES. In no event shall any fractional share of Common
Stock be issued upon any exercise of this Warrant. If, upon exercise of this
Warrant as an entirety, the Holder would, except as provided in this Section 12,
be entitled to receive a fractional share of Common Stock, then the Company
shall issue the next higher number of full shares of Common Stock, issuing a
full share with respect to such fractional share.

         13. CERTIFICATE OF ADJUSTMENT. Whenever the Purchase Price is adjusted,
as herein provided, the Company shall promptly deliver to the Holder a
certificate of a firm of independent public accountants setting forth the
Purchase Price after such adjustment and setting forth a brief statement of the
facts requiring such adjustment.

         14. NOTICES OF RECORD DATE, ETC. In the event of:

                  (a) any taking by the Company of a record of the holders of
any class of securities for the purpose of determining the holders thereof who
are entitled to receive any dividend or other distribution, or any right to
subscribe for, purchase or otherwise acquire any shares of stock of any class or
any other securities or property, or to receive any other right,

                  (b) any reclassification of the capital stock of the Company,
capital reorganization of the Company, consolidation or merger involving the
Company, or sale or conveyance of all or substantially all of its assets, or

                  (c) any voluntary or involuntary dissolution, liquidation or
winding-up of the Company,

then and in each such event the Company will mail or cause to be mailed to the
Holder a notice specifying (i) the date on which any such record is to be taken
for the purpose of such dividend, distribution or right, and stating the amount
and character of such dividend, distribution or right, or (ii) the date on which
any such reclassification, reorganization, consolidation, merger, sale or
conveyance, dissolution, liquidation or winding-up is to take place, and the
time, if any is to be fixed, as of which the holders of record in respect of
such event are to be determined. Such notice shall be mailed at least twenty
(20) days prior to the date specified in such notice on which any such action is
to be taken.

<PAGE>

                                      -5-

         15. AMENDMENT. The terms of this Warrant may be amended, modified or
waived only with the written consent of the Company and the Holder hereof.

         16. WARRANT REGISTER; TRANSFERS, ETC.

                  A. The Company will maintain a register containing the names
and addresses of the registered holders of the Warrant. The Holder may change
its address as shown on the warrant register by written notice to the Company
requesting such change. Any notice or written communication required or
permitted to be given to the Holder may be given by certified mail or delivered
to the Holder at its address as shown on the warrant register.

                  B. Subject to compliance with applicable federal and state
securities laws, this Warrant may be transferred by the Holder with respect to
any or all of the shares purchasable hereunder. Upon surrender of this Warrant
to the Company, together with the assignment hereof properly endorsed, for
transfer of this Warrant as an entirety by the Holder, the Company shall issue a
new warrant of the same denomination to the assignee. Upon surrender of this
Warrant to the Company, together with the assignment hereof properly endorsed,
by the Holder for transfer with respect to a portion of the shares of Common
Stock purchasable hereunder, the Company shall issue a new warrant to the
assignee, in such denomination as shall be requested by the Holder hereof, and
shall issue to such Holder a new warrant covering the number of shares in
respect of which this Warrant shall not have been transferred.

                  C. In case this Warrant shall be mutilated, lost, stolen or
destroyed, the Company shall issue a new warrant of like tenor and denomination
and deliver the same (i) in exchange and substitution for and upon surrender and
cancellation of any mutilated Warrant, or (ii) in lieu of any Warrant lost,
stolen or destroyed, upon receipt of evidence reasonably satisfactory to the
Company of the loss, theft or destruction of such Warrant (including a
reasonably detailed affidavit with respect to the circumstances of any loss,
theft or destruction) and of indemnity reasonably satisfactory to the Company.

         17. CERTAIN OBLIGATIONS. This Warrant has been issued subject to
certain obligations to register the Common Stock under the Securities Act of
1933, as amended, which obligations are set forth in the Agreement, of even date
herewith, between the Company, and Paramount, which are incorporated herein by
reference.

         18. NO IMPAIRMENT. The Company will not, by amendment of its
certificate of incorporation, as amended or through any reclassification,
capital reorganization, consolidation, merger, sale or conveyance of assets,
dissolution, liquidation, issue or sale of securities or any other voluntary
action, avoid or seek to avoid the observance or performance of any of the terms
of this Warrant, but will at all times in good faith assist in the carrying out
of all such terms and in the taking of all such action as may be necessary or
appropriate in order to protect the rights of the Holder.

         19. GOVERNING LAW. The provisions and terms of this Warrant shall be
governed by and construed in accordance with the internal laws of the
Commonwealth of Massachusetts.

<PAGE>

                                      -6-

         20. SUCCESSORS AND ASSIGNS. This Warrant shall be binding upon the
Company's successors and assigns and shall inure to the benefit of the Holder's
successors, legal representatives and permitted assigns.

         21. BUSINESS DAYS. If the last or appointed day for the taking of any
action required or the expiration of any right granted herein shall be a
Saturday or Sunday or a legal holiday in the Commonwealth of Massachusetts, then
such action may be taken or right may be exercised on the next succeeding day
which is not a Saturday or Sunday or such a legal holiday.

Dated: March 9, 2000                                REPLIGEN CORPORATION



(Corporate Seal)                                    By:
                                                       -------------------------
                                                      Name: Walter Herlihy
Attest:                                               Title: President and CEO



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

<PAGE>

                                      -7-

                                  Subscription


To:                                              Date:
   ---------------------------                        --------------------------


         The undersigned hereby subscribes for __________ shares of Common Stock
covered by this Warrant. The certificate(s) for such shares shall be issued in
the name of the undersigned or as otherwise indicated below:


                                                 -------------------------------
                                                 Signature


                                                 -------------------------------
                                                 Name for Registration


                                                 -------------------------------
                                                 Mailing Address



                            Net Issue Election Notice


To:                                              Date:
  -----------------------------                       --------------------------

         The undersigned hereby elects under Section 4 to surrender the right to
purchase _______ shares of Common Stock pursuant to this Warrant. The
certificate(s) for the shares issuable upon such net issue election shall be
issued in the name of the undersigned or as otherwise indicated below.


                                                 -------------------------------
                                                 Signature


                                                 -------------------------------
                                                 Name for Registration


                                                 -------------------------------
                                                 Mailing Address

<PAGE>

                                      -8-

                                   Assignment


         For value received ____________________________ hereby sells,

assigns and transfers unto _____________________________________________________

________________________________________________________________________________
    Please print or typewrite name and address of Assignee

________________________________________________________________________________

the within Warrant, and does hereby irrevocably constitute and appoint
_______________________ its attorney to transfer the within Warrant on the books
of the within named Company with full power of substitution on the premises.

Dated:_______________________


                                                ________________________________

In the Presence of:


_____________________________