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                                   EXHIBIT 4.2

                                                                   March 2, 2000

Walter C. Herlihy, Ph.D.
Chief Executive Officer
Repligen Corporation
117 Fourth Avenue
Needham, MA 02494

                                FINDERS AGREEMENT

Dear Sirs:

         Reference is made to our recent discussions relating to a proposed
private placement under Rule 506 of Regulation D of the Securities Act of 1933,
as amended (the "Act") of securities of Repligen Corp. (the "Company") as
hereinafter described. Based upon our discussions and representations which you
have made to us describing the Company and its principals, the present and
proposed business activities of the Company and the Company's operations and
financial condition, Paramount Capital, Inc. ("Paramount") hereby confirms in
principle its interest in acting as a finder for the Company, on a "best
efforts" basis, in connection with the private placement offering of the
Company's shares of Common Stock (the "Offering"), upon the following basic
terms and conditions:

         1. Paramount will introduce the Company to "accredited investors" as
defined in Rule 501 of Regulation D promulgated under the Act for the purchase
of up to 2,650,000 shares of common stock, par value $.01 per share (the "Common
Stock"). Each share of Common Stock will be sold at a price per share price (the
"Per Share Price") equal to eighty five percent (85%) of the average closing bid
price of the Common Stock for the fifteen (15) trading days immediately
preceding the Closing Date (as defined in paragraph 2 below) of the Offering;
provided, however, that in no event will the Per Share Price be greater than
$8.625. Further, subject to the Break-up Fee (as defined in paragraph 20 below)
the Company shall not be required to sell the Common Stock at a Price Per Share
less than $6.00. For purposes hereof, "closing bid price" shall mean, for each
trading day, the price at which the Common Stock was last exchanged on the
Nasdaq National Market during such trading day, or, if there were no
transactions on such trading day, the average of the reported closing bid and
asked prices, regular way, of the Common Stock on such trading day. "Trading
day" shall mean a day on which the relevant Nasdaq National Market is open for
the transaction of business.

         2. The closing date of the Offering will occur no later than sixty (60)
days following the execution of this Agreement, subject to extension at the
option of the Company for an additional thirty (30) days (the date of any such
closing is hereinafter referred to as the


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"Closing Date"). Pending completion or termination (pursuant to paragraph 18
below) of the Offering, the Company agrees that it will not enter into any
agreement, discussion or negotiations with any other person or entity relating
to a possible private offering or placement of its securities.

         3. Following (a) execution of a Securities Purchase Agreement to be
entered into by and among the Company and each purchaser (the "Purchase
Agreement") and (b) receipt by the Company of the Offering Amount, the Company
may conduct a Closing. Paramount agrees to use its reasonable best efforts to
assist the Company in causing a Closing to occur as soon as practicable
following execution of Purchase Agreements by the parties thereto. Prior to such
Closing, all amounts shall be held in escrow for the benefit of investors by an
escrow agent reasonably acceptable to the Company and Paramount.

         4. (a) Subject to the terms herein, the Company will, as soon as
practicable, but not later than 30 days after the Closing Date (the "Outside
Filing Date") (a) file a shelf registration statement (the "Shelf Registration
Statement") with respect to (i) the resale of the shares of Common Stock sold in
the Offering and (ii) the shares of Common Stock issuable upon exercise of the
Paramount Warrants (as defined below) and (together, the "Registrable Capital
Stock") with the SEC and use its best efforts to have such Shelf Registration
Statement declared effective by the SEC prior to the date which is 90 days after
the Closing Date and (b) cause such Shelf Registration Statement to remain
effective until the earlier of (a) such date as the holders of the securities
have completed the distribution described in the Shelf Registration Statement or
at such time that such shares are no longer, by reason of Rule 144(k) under the
Securities Act, required to be registered for the sale thereof by such holders
and (b) two years from the effective date of such Shelf Registration Statement.

         (b) Subject to the terms herein, in the event that the Shelf
Registration Statement is not filed by the Outside Filing Date the Company
shall, for no additional consideration, pay to each investor as liquidated
damages and not as a penalty an amount in cash equal to one percent (1%) of the
amount invested by such investor for each 30 day period in which the Shelf
Registration Statement remains unfiled; PROVIDED, HOWEVER, that in no event
shall the amount of liquidated damages payable by the Company to any investor
exceed twelve (12%) of the amount invested by such investor.

     (c) The Outside Filing Date shall be extended by at least 5 business days
from the date that the Company has received all such information that the
Company has requested in writing for use in the Shelf Registration Statement;
provided that the Company agrees to request such information within 10 days from
the Closing Date.

     5. Prior to the effective date of the Shelf Registration Statement, the
Company will file a listing application for (a) the shares of Common Stock sold
in the Offering and (b) the Common Stock issuable upon exercise of the Paramount
Warrants (as defined below) with the Nasdaq National Market.



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     6. Paramount will receive cash commissions equal to seven percent (7%) of
the aggregate gross proceeds received by the Company in the Offering (the "Cash
Commissions"). Paramount may, in its discretion, retain others, who shall be
members in good standing of the National Association of Securities Dealers, Inc.
("NASD"), to act as selected dealers in placing the Common Stock. Such other
others will be compensated by Paramount out of its commissions. The Company has
advised Paramount that no person is entitled, directly or indirectly, to
compensation from the Company for services as a finder in connection with the
proposed Offering or any other transaction contemplated by this Finders
Agreement.

     7. Pending completion of the Offering and for a 30 day period thereafter,
the Company will not issue press releases with respect to the Offering or engage
in other publicity without giving Paramount advance copies of any such releases.
Other than as a result of stock splits, during the 12 month period following the
completion of the Offering, the Company will not lower the exercise price or
conversion price of any options, warrants or convertible securities currently
outstanding without the prior written consent of Paramount. The Company shall
not use the name of Paramount or any officer, director, employee or shareholder
without the express written consent of Paramount and such person other than as
required by applicable law, the rules and regulations of the United States
Securities and Exchange Commission and the NASD.

     8. The Company shall be responsible for and shall bear all out of pocket
expenses incurred by Paramount in connection with the Offering including the
Paramount's legal fees, such fees and expenses not to exceed $75,000 (the
"Expense Allowance"). The Company shall pay to Paramount an amount equal to
$5,000 as a retainer, which shall be due and payable upon execution of this
Finder's Agreement and which shall be creditable against the Expense Allowance,
to cover the cost of the Paramount's mailing, telephone, telegraph, travel, due
diligence meetings and other similar expenses including legal fees of
Paramount's counsel (other than legal fees in connection with blue sky matters
as to which fees the Company shall be responsible). Such prepaid expense
allowance shall be non-refundable.

     9. Upon consummation of the Offering contemplated hereby, the Company will,
in consideration of the services rendered by Paramount in connection with the
Offering, issue to Paramount and/or its designees for no additional
consideration (provided that such issuances shall in all events be made in
compliance with applicable securities laws, including compliance with the rules
governing exemption from registration requirements of the Act), warrants (the
"Paramount Warrants") to acquire a number of shares of Common Stock equal to
five percent (5%) of the number of shares of Common Stock sold in the Offering,
exercisable for a period of five (5) years commencing six (6) months after the
closing of the Offering at an exercise price equal to 110% of the price per
share of Common Stock sold in the Offering. The Paramount Warrants cannot be
transferred, sold, assigned or hypothecated for six (6) months except that they
may be assigned in whole or in part during such period to any NASD member
participating in the Offering or any officer or employee of Paramount or any
such NASD member. The Paramount Warrants will contain a cashless exercise
feature, anti-dilution protection and the right to have the Common Stock
issuable upon exercise thereof (including the Warrants issuable upon exercise of
the Paramount Warrants) included on the Shelf Registration



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Statement. For purposes of clarification, the Company agrees that for a period
of 12 months from the Closing Date, the Paramount Warrants shall provide for
protection against dilution as a result of issuances by the Company of common
stock, or securities convertible into or exercisable for common stock, at a
price per share less than the average closing bid price for the five (5) days
preceding any such issuance.

     10. Paramount shall be entitled to receive the Cash Commissions described
in paragraph 6 and the Paramount Warrants described in paragraph 9 in the event
that any investor (a) who is or has been introduced to the Company by Paramount
and (b) who has been identified in writing to the Company prior to making an
investment and (c) with whom Paramount has had discussions regarding an
investment in the Company makes an investment in the Company during the 12-month
period following the Closing Date. Paramount will provide the Company with a
list of such introduced investors on the Closing Date. In addition, the Company
shall in all events be responsible for Paramount's reasonable out of pocket
expenses in connection with any such investment.

     11. The Company shall not use any proceeds from the Offering to repay any
indebtedness of the Company, including, but not limited to, any indebtedness to
current executive officers or principal stockholders of the Company, but
excluding accounts payable incurred in the ordinary course.

     12. (a) Unless required by law, any services and advice rendered by
Paramount pursuant to this Agreement (and the existence of this Agreement) shall
not be disclosed publicly in any manner without Paramount's prior written
approval and shall be treated by the Company as confidential information.

         (b) Confidential Information shall mean any information disclosed by
one party to the other party hereunder (in writing or verbally, whether or not
specifically marked confidential), including, without limitation, the terms of
this Agreement, all commercially valuable, proprietary or confidential
information with respect to the Company's business, products or operations
disclosed by the Company to Paramount, which includes financial information,
marketing plans, strategies and other valuable business information.
Confidential Information shall also include any financial advice provided
hereunder by Paramount to the Company. Notwithstanding anything to the contrary
in this Section 12, Confidential Information shall not include: (i) information
that is in or enters the public domain without breach of this Agreement; (ii)
information lawfully received from a third party without restriction on
disclosure and without breach of a nondisclosure obligation; (iii) information
that is developed independently by a party which party can prove with written
evidence; and (iv) information that is required by judicial or administrative
order or by governmental authority to be disclosed, provided that the party from
whom disclosure is sought uses reasonable efforts to notify the other of such
requirement so that the other may seek to make such disclosure subject to a
protective order or confidentiality agreement.


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         (c) The Company agrees to limit disclosure of the Confidential
Information only to those of its officers, employees, agents, affiliates and
consultants as the Company considers necessary, provided that each such officer,
employee, agent, affiliate or consultant of the Company shall be similarly bound
by the confidentiality obligations contained herein. The Company shall take all
reasonable commercial steps to prevent the disclosure of the Confidential
Information as it would to protect its own confidential or proprietary
information. This obligation shall be binding upon the Company and shall
continue for a period during the term of this Agreement and for a period of five
(5) years thereafter.

         (d) Paramount agrees to limit disclosure of the Confidential
Information only to those of its officers, employees, agents, affiliates and
consultants as Paramount considers necessary to render its services under this
Agreement, provided that each such officer, employee, agent, affiliate or
consultant of Paramount shall be similarly bound by the confidentiality
obligations contained herein. Paramount shall take all reasonable steps to
prevent the disclosure of the Confidential Information as it would to protect
its own confidential or proprietary information. Paramount understands and
agrees that to the extent Paramount receives Confidential Information which is
material and non-public relating to the Company, Paramount shall be deemed to be
an "insider" for purposes of United States securities laws and shall be
prohibited from selling, purchasing or otherwise trading in the securities of
the Company until public disclosure of such material non-public information.
Paramount may rely, without independent verification, on the accuracy and
completeness of any written information furnished to Paramount by the Company,
subject to its obligations under the securities laws and herein. This obligation
shall be binding upon Paramount and shall continue for a period during the term
of this Agreement and for a period of five (5) years thereafter.

     13. Paramount shall be entitled to rely on the representations, warranties
and covenants of the Company as set forth in any Purchase Agreement or other
document used by the Company in connection with or otherwise related to the
Offering, including without limitation the legal opinion of counsel to the
Company. Further, Paramount reserves the right to conduct legal, business and
financial due diligence of the Company to the extent that Paramount, in its sole
discretion, deems it necessary and appropriate.

     14. (a) The Company agrees to indemnify and hold harmless Paramount and its
respective partners, shareholders, directors, officers, agents, advisors,
representatives, employees, counsel and controlling persons within the meaning
of the Act (a "Paramount Indemnified Party") against any and all losses,
liabilities, claims, damages and expenses whatsoever (and all actions in respect
thereof), and to promptly reimburse any such Paramount Indemnified Party for the
reasonable legal fees and related expenses as incurred (including, but not
limited to the costs of giving testimony or furnishing documents in response to
a document request, a subpoena or otherwise, the costs of investigating,
preparing, pursuing or defending any such action or claim whether or not pending
or threatened and whether or not any Paramount Indemnified Party is a party
thereto), insofar as such losses, liabilities, claims, damages or expenses arise
out of, relate to, are in incurred in connection with or are in any way a result
of or relate to this Agreement or the matters contemplated by this Agreement;
PROVIDED HOWEVER that



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the Company shall not be liable to a Paramount Indemnified Party hereunder to
the extent that any loss, claim, damage, liability or expense is found in final
judgment by a court or arbitrator to have resulted from a Paramount Indemnified
Party's malfeasance, bad faith or negligence; and PROVIDED FURTHER that in no
event shall the Company's liability under this Section 14 exceed the aggregate
amount of fees paid by the Company to Paramount hereunder.

         (b) Paramount agrees to indemnify and hold harmless the Company and its
respective partners, shareholders, directors, officers, agents, advisors,
representatives, employees, counsel and controlling persons within the meaning
of the Securities Act (a "Company Indemnified Party") against any and all
losses, liabilities, claims, damages and expenses whatsoever (and all actions in
respect thereof), and to promptly reimburse any such Company Indemnified Party
for reasonable legal fees and related expenses as incurred (including, but not
limited to the costs of giving testimony or furnishing documents in response to
a document request, a subpoena or otherwise, the costs of investigating,
preparing, pursuing or defending any such action or claim whether or not pending
or threatened and whether or not any Paramount Indemnified Party is a party
thereto), insofar as such losses, liabilities, claims, damages or expenses arise
out of, relate to, are in incurred in connection with or are in any way a result
of or relate to Paramount's gross negligence or willful misconduct in the
performance of the services pursuant to this Agreement; PROVIDED HOWEVER that
Paramount shall not be liable hereunder to the extent that any loss, claim,
damage, liability or expense is found in final judgment by a court or arbitrator
to have resulted from a Company Indemnified Party's malfeasance, bad faith or
negligence; and PROVIDED FURTHER that in no event shall Paramount's liability
under this Section 14 exceed the aggregate amount of fees paid by the Company to
Paramount hereunder.

         (c) The indemnification provisions of this Section 14 shall be the sole
and exclusive remedies of the parties hereto with respect to any claims arising
hereunder by one party against the other party hereunder, except that any claim
by one party against the other party under this Agreement for an intentional and
material breach of Section 12 hereunder or a claim based on fraud, bad faith or
gross negligence shall not be limited to the remedies and limitations of this
Section 14.

     15. This Agreement shall continue for a term of sixty (60) days from the
date hereof, subject to the right of the Company to extend the term for up to an
additional thirty (30) day period, provided that good-faith negotiations are
continuing with investors to complete the Offering. Notwithstanding the
foregoing, Paramount may terminate this Agreement at any time prior to the
closing of the Offering in its sole discretion, with or without cause, and
without liability whatsoever to the Company. Paragraphs 8, 10, 12, 13, 14, 15,
16, 18, 19 and 20of this Agreement shall remain operative and in full force and
effect regardless of any expiration or termination of this Agreement by the
Company.



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     16. This Agreement shall be governed by and construed in accordance with
the laws of the State of New York without regard to principles of conflicts of
law. The parties hereby irrevocably submit to the exclusive jurisdiction of the
Courts of the State of New York.

     17. This Agreement shall be binding upon and inure to the benefit of
Paramount and the Company and each of their successors and assigns. This
Agreement may not be assigned by either party without the prior written consent
of the other.

     18. Paramount may, in its sole judgment and discretion, determine to
terminate this Agreement. The Company may in its sole judgment and discretion,
determine at any time not to proceed with the Offering; provided, however, that
in the event of such a termination, the Company shall pay to Paramount a fee
(the "Break-up Fee") equal to $100,000 (in addition to Paramount's reasonable
out of pocket expenses (for which the Company shall in all events remain
liable)).

     19. Nothing herein shall restrict or otherwise limit Paramount from
performing similar or dissimilar services for any other party or for its own
account. The provisions of this paragraph 19 shall be enforceable to the fullest
extent permitted by law.

     20. This Agreement embodies the entire agreement and understanding between
the parties hereto and supersedes any prior agreements or understandings, oral
or written, relating to the subject matter hereof. If any provision of this
Agreement is determined to be invalid or unenforceable in any respect, such
determination will not affect such provision in any other respect or any other
provision of this Agreement, which will remain in full force and effect. This
Agreement may not be amended or otherwise modified or waived except by an
instrument in writing signed by both the Company and Paramount.



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If the foregoing conforms to your understanding, please sign, date and return to
us the enclosed copy of this letter.

                                     Very truly yours,

                                     PARAMOUNT CAPITAL, INC.

                                     By:  /S/ LINDSAY ROSENWALD
                                          ---------------------------------
                                     Name:    Lindsay A. Rosenwald, M.D.
                                     Title:   Chairman

The foregoing is in conformity with our understanding:

REPLIGEN CORP.

By:  /S/ WALTER C. HERLIHY
     -----------------------------------------
Name:    Dr. Walter C. Herlihy
Title:   President and Chief Executive Officer




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