REPLIGEN CORP
S-3, 2000-05-04
BIOLOGICAL PRODUCTS, (NO DIAGNOSTIC SUBSTANCES)
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<PAGE>


            AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON MAY 4, 2000
                                                    REGISTRATION NO. ___________
================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                              WASHINGTON, DC 20549

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

                                    FORM S-3
                             REGISTRATION STATEMENT
                                      UNDER
                           THE SECURITIES ACT OF 1933

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

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

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

<TABLE>

<S>                                     <C>                                     <C>
           DELAWARE                                2836                               04-2729386
   (State or other jurisdiction          (Primary Standard Industrial                (I.R.S. Employer
 of incorporation or organization)        Classification Code Number)             Identification Number)
</TABLE>

                                117 FOURTH AVENUE
                                NEEDHAM, MA 02494
                                 (781) 449-9560
               (Address, including zip code, and telephone number,
        including area code, of Registrant's principal executive offices)

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

                                WALTER C. HERLIHY
                      PRESIDENT AND CHIEF EXECUTIVE OFFICER
                              REPLIGEN CORPORATION
                                117 FOURTH AVENUE
                                NEEDHAM, MA 02494
                                 (781) 449-9560
            (Name, address, including zip code, and telephone number,
                   including area code, of agent for service)

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

  COPIES OF ALL COMMUNICATIONS, INCLUDING ALL COMMUNICATIONS SENT TO THE AGENT
                         FOR SERVICE, SHOULD BE SENT TO:

                          LAWRENCE S. WITTENBERG, ESQ.
                         Testa, Hurwitz & Thibeault, LLP
                                High Street Tower
                                 125 High Street
                           Boston, Massachusetts 02110
                                 (617) 248-7000

<TABLE>
<CAPTION>

                                          CALCULATION OF REGISTRATION FEE
- -------------------------------------- --------------- ------------------------- -------------------------- ----------------
                                                           PROPOSED MAXIMUM          PROPOSED MAXIMUM          AMOUNT OF
    TITLE OF EACH CLASS OF SECURITIES   AMOUNT TO BE       OFFERING PRICE PER        AGGREGATE OFFERING       REGISTRATION
             TO BE REGISTERED            REGISTERED            SHARE(1)                  PRICE(1)               FEE(2)
- -------------------------------------- --------------- ------------------------- -------------------------- ----------------

<S>                                   <C>                  <C>                     <C>                       <C>
Common Stock, $0.01 par value
   per share (2)                         2,598,927              $6.25                   $16,243,294             $4,289
- -------------------------------------- --------------- ------------------------- -------------------------- ----------------
Common Stock, $0.01 par value
   per share (upon                         129,946              $6.25                      $812,163               $215
   exercise of a warrant) (2)
- -------------------------------------- --------------- ------------------------- -------------------------- ----------------
</TABLE>

(1)  Estimated solely for the purpose of calculating the registration fee
     pursuant to Rule 457 under the Securities Act of 1933, as amended.

(2)  Pursuant to Rule 457(c) under the Securities Act of 1933, the registration
     fee has been calculated based upon the average of the high and low prices
     per share of the common stock of Repligen Corporation on the Nasdaq
     National Market on May 1, 2000.

APPROXIMATE DATE OF COMMENCEMENT OF PROPOSED SALE TO THE PUBLIC: As soon as
practicable after this Registration Statement becomes effective.
     If the only securities being registered on this form are being offered
pursuant to dividend or interest reinvestment plans, please check the following
box. / /
     If any of the securities being registered on this Form are to be offered on
a delayed or continuous basis pursuant to Rule 415 under the Securities Act of
1933, other than securities offered only in connection with dividend or interest
reinvestment plans, check the following box. /X/
     If this Form is filed to register additional securities for an offering
pursuant to Rule 462(b) under the Securities Act, please check the following box
and list the Securities Act registration statement number of the earlier
effective registration statement for the same offering. / /
     If this Form is a post-effective amendment filed pursuant to Rule 462(c)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering. / /
     If delivery of the prospectus is expected to be made pursuant to Rule 434,
please check the following. / /

     REPLIGEN HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE OR DATES AS
MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL REPLIGEN SHALL FILE A FURTHER
AMENDMENT WHICH SPECIFICALLY STATES THAT THIS REGISTRATION STATEMENT SHALL
THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH SECTION 8(a) OF THE SECURITIES
ACT OF 1933 OR UNTIL THE REGISTRATION STATEMENT SHALL BECOME EFFECTIVE ON SUCH
DATE AS THE COMMISSION, ACTING PURSUANT TO SECTION 8(a), MAY DETERMINE.

================================================================================

<PAGE>


THE INFORMATION IN THIS PROSPECTUS IS NOT COMPLETE AND MAY BE CHANGED. WE MAY
NOT SELL THESE SECURITIES UNTIL THE REGISTRATION STATEMENT FILED WITH THE
SECURITIES AND EXCHANGE COMMISSION IS EFFECTIVE. THIS PROSPECTUS IS NOT AN OFFER
TO SELL SECURITIES, AND IT IS NOT SOLICITING OFFERS TO BUY THESE SECURITIES IN
ANY STATE WHERE THE OFFER OR SALE IS NOT PERMITTED.


<PAGE>


                    SUBJECT TO COMPLETION, DATED MAY 4, 2000






                              REPLIGEN CORPORATION



                                2,728,873 SHARES

                                  COMMON STOCK





         The selling stockholders are offering for sale up to 2,598,927 shares
of our common stock and one of the selling stockholders is offering for sale up
to 129,946 shares of our common stock upon the exercise of a warrant held by it.

         Repligen's common stock is traded on the Nasdaq National Market under
the symbol "RGEN." The last reported sale price of our common stock on the
Nasdaq National Market on May 1, 2000 was $6.25 per share.












                  INVESTING IN THE COMMON STOCK INVOLVES RISKS.
                     SEE "RISK FACTORS" BEGINNING ON PAGE 3.




         NEITHER THE SECURITIES AND EXCHANGE COMMISSION NOR ANY STATE SECURITIES
COMMISSION HAS APPROVED OR DISAPPROVED THESE SECURITIES, OR DETERMINED IF THIS
PROSPECTUS IS TRUTHFUL OR COMPLETE. ANY REPRESENTATION TO THE CONTRARY IS A
CRIMINAL OFFENSE.





                  The date of this prospectus is May ___, 2000.


<PAGE>


YOU SHOULD RELY ONLY ON THE INFORMATION CONTAINED IN THIS PROSPECTUS. WE HAVE
NOT AUTHORIZED ANYONE TO PROVIDE YOU WITH INFORMATION DIFFERENT FROM THAT
CONTAINED IN THIS PROSPECTUS. THE INFORMATION CONTAINED IN THIS PROSPECTUS IS
ACCURATE ONLY AS OF THE DATE OF THIS PROSPECTUS, REGARDLESS OF THE TIME OF
DELIVERY OF THIS PROSPECTUS OR OF ANY SALE OF THE COMMON STOCK.

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

                                TABLE OF CONTENTS
                                                                           PAGE
Repligen..............................................................        2
Risk Factors..........................................................        3
Legal Proceedings.....................................................       10
Use of Proceeds.......................................................       10
Selling Stockholders..................................................       11
Plan of Distribution..................................................       17
Legal Matters.........................................................       18
Experts...............................................................       18
Where You Can Find More Information...................................       18

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


<PAGE>



                                    REPLIGEN

     We develop new drugs for autism, organ transplantation and cancer. To
expand our drug development program, on March 9, 1999, we acquired the exclusive
rights to patent applications for the use of secretin in the treatment of
autism. Autism is a developmental disorder characterized by poor communicative
and social skills, repetitive and restricted behaviors and in some patients,
gastrointestinal problems and irregular sleep patterns. Secretin is a hormone
produced in the small intestine which regulates the function of the pancreas as
part of the process of digestion. A form of secretin derived from pigs is
approved by the FDA for use in diagnosing problems with pancreatic function.
Recent anecdotal reports indicate that secretin may have beneficial effects in
autism, including improvements in sleep, digestive function, communicative and
social behavior. Following media reports of the potential benefits of secretin,
more than 2,000 autistic children have been treated with the pig-derived
hormone. We intend to manufacture a human, synthetic form of secretin and
evaluate it in FDA approved clinical trials in order to confirm the benefits of
secretin in treating autism and to determine the optimal dosing schedule. There
are currently no drugs approved by the FDA for the treatment of autism.

     In October 1999, we obtained a license for the commercialization rights to
two diagnostic secretin products from ChiRhoClin Inc., a private company. These
products have been evaluated in clinical trials for the diagnosis of pancreatic
dysfunction and gastrinoma. A New Drug Application was filed with the FDA in May
1999 seeking approval to market synthetic porcine secretin for these
applications. ChiRhoClin has also conducted clinical studies for these
diagnostic indications with a human form of secretin. Under terms of the
agreement, we made an upfront payment upon execution of the agreement and, if
the FDA approves the New Drug Applications, the agreement obligates us to pay
ChiRhoClin future milestones and royalties.

     We are also developing a product named "CTLA4-Ig," which has been shown to
suppress unwanted immune responses in animal models of organ transplants and
autoimmune diseases, such as lupus or multiple sclerosis, in which the immune
system mistakenly attacks the body. Our product candidate is a derivative of a
natural protein whose role is to turn-off an immune response. In animal models
of organ transplantation and autoimmune diseases, CTLA4-Ig has been shown to
block the rejection of a transplanted organ or the effects of the autoimmune
disease. Initial clinical testing of CTLA4-Ig has been carried out in patients
receiving a bone marrow transplant, which is a potential cure for several
diseases of the immune system, including leukemia, myeloma, lymphoma and sickle
cell anemia. Despite the clinical success of bone marrow transplants, a
significant number of patients experience a severe and potentially
life-threatening complication known as Graft Versus Host Disease, in which the
newly transplanted immune system attacks the host (i.e., the patient). In June
1999, results from a Phase 1 clinical trial reported that treatment of bone
marrow from a family member with Repligen's CTLA4-Ig prevented Graft Versus Host
Disease in eight of eleven transplant patients. In September 1999, we signed a
Clinical Trial Agreement with the National Cancer Institute to further evaluate
CTLA4-Ig in a Phase 2 trial in bone marrow transplantation for leukemia.
Repligen has filed patent applications related to compositions of matter and
methods of use of CTLA4-Ig including bone marrow transplantation. Certain
patents have been issued to Bristol-Myers Squibb Corporation relating to the use
and manufacture of CTLA4-Ig. We believe that one of our

                                       -2-

<PAGE>

licensees is the co-inventor of one or more of these patents and that the
patents issued to Bristol-Myers Squibb do not extend to our use of CTLA4-Ig in
bone marrow transplantation. For more information on our patent litigation,
please see "Legal Proceedings".

     We also develop, manufacture and market products for the production of
therapeutic antibodies. We currently market a line of products for the
purification of antibodies based on a naturally occurring protein, Protein A,
which can specifically bind to antibodies. We own composition of matter patents
for recombinant Protein A in the United States and in Europe. In December 1998,
we entered into a ten year agreement to supply recombinant Protein A to Amersham
Pharmacia Biotech, a leading supplier to the biopharmaceutical market.

     Our executive offices are located at 117 Fourth Avenue, Needham,
Massachusetts 02494, and our telephone number is (781) 449-9560.

                                  RISK FACTORS

     YOU SHOULD CAREFULLY CONSIDER THE RISKS DESCRIBED BELOW BEFORE MAKING AN
INVESTMENT DECISION.

     IF ANY OF THE FOLLOWING RISKS OCCUR, OUR BUSINESS, FINANCIAL CONDITION OR
RESULTS OF OPERATIONS COULD BE MATERIALLY HARMED. IN THAT CASE THE TRADING PRICE
OF OUR COMMON STOCK COULD DECLINE, AND YOU MAY LOSE ALL OR PART OF YOUR
INVESTMENT.

     THIS PROSPECTUS ALSO CONTAINS FORWARD LOOKING STATEMENTS THAT INVOLVE RISKS
AND UNCERTAINTIES. OUR ACTUAL RESULTS COULD DIFFER MATERIALLY FROM THOSE
ANTICIPATED IN THESE FORWARD LOOKING STATEMENTS AS A RESULT OF CERTAIN FACTORS,
INCLUDING THE RISKS FACED BY US DESCRIBED BELOW AND ELSEWHERE IN THIS
PROSPECTUS.

WE MAY BE DEPENDENT ON OUR COLLABORATIVE PARTNERS TO DEVELOP, CONDUCT CLINICAL
TRIALS FOR, AND MANUFACTURE, MARKET AND SELL OUR PRINCIPAL PRODUCTS.

     We conduct some of our development activities, and may conduct most of our
commercialization activities, through collaborations. Our collaborations are
heavily dependent on the efforts and activities of our collaborative partners.
Our existing and any future collaborations may not be scientifically or
commercially successful.

     For example, if any of our collaborative partners were to breach or
terminate an agreement with us, reduce its funding or otherwise fail to conduct
the collaboration successfully, we may need to devote additional internal
resources to the program that is the subject of the collaboration, scale back or
terminate the program or seek an alternative partner.


                                      -3-

<PAGE>

THE MARKET MAY NOT BE RECEPTIVE TO OUR PRODUCTS UPON THEIR INTRODUCTION.

     The commercial success of our products that are approved for marketing will
depend upon their acceptance by the medical community and third party payors as
being clinically useful, cost effective and safe. All of the products that we
are developing are based upon new technologies or therapeutic approaches. As a
result, it is hard to predict market acceptance of our products.

     Other factors that we believe will materially affect market acceptance of
our products and services include:

- -    the timing of receipt of marketing approvals and the countries in which
     such approvals are obtained;

- -    the safety, efficacy and ease of administration of our products;

- -    the success of physician education programs; and

- -    the availability of government and third party payor reimbursement of our
     products.

WE COMPETE WITH LARGER, BETTER FINANCED AND MORE MATURE PHARMACEUTICAL AND
BIOTECHNOLOGY COMPANIES WHO ARE CAPABLE OF DEVELOPING NEW APPROACHES THAT COULD
MAKE OUR PRODUCTS AND TECHNOLOGY OBSOLETE.

     The market for therapeutic and bioprocessing products is intensely
competitive, rapidly evolving and subject to rapid technological change.
Pharmaceutical and mature biotechnology companies have substantially greater
financial, manufacturing, marketing, research and development resources than we
have. New approaches to the treatment of our targeted diseases by these
competitors may make our products and technologies obsolete or noncompetitive.

WE HAVE INCURRED SUBSTANTIAL LOSSES, WE EXPECT TO CONTINUE TO INCUR LOSSES AND
WE WILL NOT BE SUCCESSFUL UNTIL WE REVERSE THIS TREND.

     We have incurred losses in each year since our founding in 1981. We expect
to continue to incur operating losses for the foreseeable future.

     While we generate revenue from product sales, this revenue is not
sufficient to cover the costs of our clinical trials and drug development
programs. We expect to increase our spending significantly as we continue to
expand our research and development programs and commercialization activities.
As a result, we will need to generate significant revenues in order to achieve
profitability. We cannot be certain whether or when this will occur because of
the significant uncertainties that affect our business.

IF WE DO NOT OBTAIN ADDITIONAL CAPITAL FOR OUR DRUG DEVELOPMENT PROGRAMS, WE
WILL BE UNABLE TO DEVELOP OR DISCOVER NEW DRUGS.

     We need additional long-term financing to develop our drug development
programs through the clinical trial process as required by the FDA and our
bioprocessing products business. We also need additional long-term financing to
support future operations and capital expenditures, including capital for
additional personnel and facilities. If we spend more money than currently
expected for


                                      -4-

<PAGE>

our drug development programs and our bioprocessing products business, we will
need to raise additional capital by selling debt or equity securities, by
entering into strategic relationships or through other arrangements. We may be
unable to raise any additional amounts on reasonable terms when they are needed
due to the volatile nature of the biotechnology marketplace. If we are unable to
raise this additional capital, we may have to delay or postpone critical
clinical studies or abandon other development programs.

IF OUR CLINICAL TRIALS ARE NOT SUCCESSFUL, WE WILL NOT BE ABLE TO DEVELOP AND
COMMERCIALIZE ANY RELATED PRODUCTS.

     In order to obtain regulatory approvals for the commercial sale of our
future products, we and our collaborative partners will be required to complete
extensive clinical trials in humans to demonstrate the safety and efficacy of
the products. We have limited experience in conducting clinical trials.

     The submission of an Investigational New Drug Application may not result in
FDA authorization to commence clinical trials. If clinical trials begin, we or
our collaborative partners may not complete testing successfully within any
specific time period, if at all, with respect to any of our products.
Furthermore, we, our collaborative partners, or the FDA, may suspend clinical
trials at any time on various grounds, including a finding that the subjects or
patients are being exposed to unacceptable health risks. Clinical trials, if
completed, may not show any potential product to be safe or effective. Thus, the
FDA and other regulatory authorities may not approve any of our potential
products for any indication.

     The rate of completion of clinical trials is dependent in part upon the
rate of enrollment of patients. Patient enrollment is a function of many
factors, including the size of the patient population, the proximity of patients
to clinical sites, the eligibility criteria for the study, and the existence of
competitive clinical trials. Delays in planned patient enrollment may result in
increased costs and program delays.

WE MAY NOT OBTAIN REGULATORY APPROVALS; THE APPROVAL PROCESS IS COSTLY AND
LENGTHY.

     We must obtain regulatory approval for our ongoing development activities
and before marketing or selling any of our future products. We may not receive
regulatory approvals to conduct clinical trials of our products or to
manufacture or market our products. In addition, regulatory agencies may not
grant such approvals on a timely basis or may revoke previously granted
approvals.

     The process of obtaining FDA and other required regulatory approvals is
lengthy and expensive. The time required for FDA and other clearances or
approvals is uncertain and typically takes a number of years, depending on the
complexity and novelty of the product. Our analysis of data obtained from
preclinical and clinical activities is subject to confirmation and
interpretation by regulatory authorities, which could delay, limit or prevent
regulatory approval. Any delay in obtaining or failure to obtain required
clearance or approvals could materially adversely affect our ability to generate
revenues from the affected product. We have only limited experience in filing
and prosecuting applications necessary to gain regulatory approvals.


                                      -5-

<PAGE>

     We also are subject to numerous foreign regulatory requirements governing
the design and conduct of the clinical trials and the manufacturing and
marketing of our future products. The approval procedure varies among countries.
The time required to obtain foreign approvals often differs from that required
to obtain FDA approvals. Moreover, approval by the FDA does not ensure approval
by regulatory authorities in other countries.

     All of the foregoing regulatory risks also are applicable to development,
manufacturing and marketing undertaken by our collaborative partners or other
third parties.

EVEN IF WE OBTAIN MARKETING APPROVAL, OUR PRODUCTS WILL BE SUBJECT TO ONGOING
REGULATORY REVIEW WHICH WILL BE EXPENSIVE AND MAY EFFECT OUR ABILITY TO
SUCCESSFULLY COMMERCIALIZE OUR PRODUCTS.

     Even if we receive regulatory approval of a product, such approval may be
subject to limitations on the indicated uses for which the product may be
marketed, which may limit the size of the market for the product or contain
requirements for costly post-marketing follow-up studies. The manufacturer of
our products for which we have obtained marketing approval will be subject to
continued review and periodic inspections by the FDA and other regulatory
authorities. The subsequent discovery of previously unknown problems with the
product, clinical trial subjects, or with the manufacturer or facility, may
result in restrictions on the product or manufacturer, including withdrawal of
the product from the market.

     If we fail to comply with applicable regulatory requirements, we may be
subject to fines, suspension or withdrawal of regulatory approvals, product
recalls, seizure of products, operating restrictions, and criminal prosecution.

IF WE ARE UNABLE TO OBTAIN AND MAINTAIN PATENTS FOR OUR PRODUCTS, WE WILL NOT BE
ABLE TO SUCCEED COMMERCIALLY.

     We must obtain and maintain patent and trade secret protection for our
products and processes in order to protect them from unauthorized use and to
produce a financial return consistent with the significant time and expense
required to bring our products to market. Our success will depend, in part, on
our ability to:

     -    obtain and maintain patent protection for our products and
          manufacturing processes;

     -    preserve our trade secrets; and

     -    operate without infringing the proprietary rights of third parties.

     We can not be sure that any patent applications relating to our products
that we will file in the future or that any currently pending applications will
issue on a timely basis, if ever. Since patent applications in the United States
are maintained in secrecy until patents issue and since publication of
discoveries in the scientific or patent literature often lag behind actual
discoveries, we cannot be certain that we were the first to make the inventions
covered by each of our pending patent applications or that we were the first to
file patent applications for such inventions. Even if patents are issued, the
degree of protection afforded by such patents will depend upon the:


                                      -6-

<PAGE>

     -    scope of the patent claims;

     -    validity and enforceability of the claims obtained in such patents;
          and

     -    our willingness and financial ability to enforce and/or defend them.

     The patent position of biotechnology and pharmaceutical firms is often
highly uncertain and usually involves complex legal and scientific questions.
Moreover, no consistent policy has emerged in the United States and in many
other countries regarding the breadth of claims allowed in biotechnology
patents. Patents which may be granted to us in certain foreign countries may be
subject to opposition proceedings brought by third parties or result in suits by
us which may be costly and result in adverse consequences for us.

     If our competitors prepare and file patent applications in the United
States that claim technology also claimed by us, we may be required to
participate in interference proceedings declared by the U.S. Patent and
Trademark Office to determine priority of invention, which would result in
substantial costs to us.

     In addition, patents blocking our manufacture, use or sale of our products
could be issued to third parties in the United States or foreign countries. The
issuance of blocking patents or an adverse outcome in an interference or
opposition proceeding, could subject us to significant liabilities to third
parties and require us to license disputed rights from third parties on
unfavorable terms, if at all, or cease using the technology.

WE MAY BECOME INVOLVED IN EXPENSIVE PATENT LITIGATION OR OTHER INTELLECTUAL
PROPERTY PROCEEDINGS WHICH COULD RESULT IN LIABILITY FOR DAMAGES OR STOP OUR
DEVELOPMENT AND COMMERCIALIZATION EFFORTS.

     There has been substantial litigation and other proceedings regarding the
complex patent and other intellectual property rights in the pharmaceutical and
biotechnology industries. We may become a party to patent litigation or other
proceedings regarding intellectual property rights.

     Other types of situations in which we may become involved in patent
litigation or other intellectual property proceedings include:

     -    We may initiate litigation or other proceedings against third parties
          to enforce our patent rights.

     -    We may initiate litigation or other proceedings against third parties
          to seek to invalidate the patents held by such third parties or to
          obtain a judgment that our products or services do not infringe such
          third parties' patents.

     -    If our competitors file patent applications that claim technology also
          claimed by us, we may participate in interference or opposition
          proceedings to determine the priority of invention.


                                      -7-

<PAGE>

     -    If third parties initiate litigation claiming that our processes or
          products infringe their patent or other intellectual property rights,
          we will need to defend against such claims.

     The cost to us of any patent litigation or other proceeding, even if
resolved in our favor, could be substantial. Some of our competitors may be able
to sustain the cost of such litigation or proceedings more effectively than we
can because of their substantially greater financial resources. If a patent
litigation or other intellectual property proceeding is resolved unfavorably to
us, we or our collaborative partners may be enjoined from manufacturing or
selling our products and services without a license from the other party and be
held liable for significant damages. We may not be able to obtain any required
license on commercially acceptable terms or at all.

     Uncertainties resulting from the initiation and continuation of patent
litigation or other proceedings could have a material adverse effect on our
ability to compete in the marketplace. Patent litigation and other proceedings
may also absorb significant management time.

WE HAVE LIMITED SALES AND MARKETING EXPERIENCE AND CAPABILITIES.

     We have limited sales, marketing and distribution experience and
capabilities. We may, in some instances, rely significantly on sales, marketing
and distribution arrangements with our collaborative partners and other third
parties. In these instances, our future revenues will be materially dependent
upon the success of the efforts of these third parties.

     If in the future we determine to perform sales, marketing and distribution
functions ourselves, we would face a number of additional risks, including:

     -    we may not be able to attract and build a significant marketing staff
          or sales force;

     -    the cost of establishing a marketing staff or sales force may not be
          justifiable in light of any product revenues; and

     -    our direct sales and marketing efforts may not be successful.

WE HAVE LIMITED MANUFACTURING CAPABILITIES AND WILL BE DEPENDENT ON THIRD PARTY
MANUFACTURERS.

     We have limited manufacturing experience and no commercial or pilot scale
manufacturing facilities for the production of pharmaceuticals. In order to
continue to develop pharmaceutical products, apply for regulatory approvals and,
ultimately, commercialize any products, we will need to develop, contract for,
or otherwise arrange for the necessary manufacturing capabilities.

     We currently rely upon third parties to produce material for preclinical
and clinical testing purposes and expect to continue to do so in the future. We
also expect to rely upon third parties, including our collaborative partners, to
produce materials required for the commercial production of certain of our
products if we succeed in obtaining necessary regulatory approvals. There are a
limited number of manufacturers that operate under the FDA's regulations for
good


                                      -8-

<PAGE>

manufacturing practices which are capable of manufacturing for us. However, if
we are unable to arrange for third party manufacturing of our products, or to do
so on commercially reasonable terms, we may not be able to complete development
of our products or market them.

     To the extent that we enter into manufacturing arrangements with third
parties, we are dependent upon these third parties to perform their obligations
in a timely manner. If such third party suppliers fail to perform their
obligations, we may be adversely affected in a number of ways, including:

     -    we may not be able to meet commercial demands for our products;

     -    we may not be able to initiate or continue clinical trials of products
          that are under development; and

     -    we may be delayed in submitting applications for regulatory approvals
          for our products.

     The manufacture of products by us and our collaborative partners and
suppliers is subject to regulation by the FDA and comparable agencies in foreign
countries. Delay in complying or failure to comply with such manufacturing
requirements could materially adversely affect the marketing of our products.

IF WE ARE UNABLE TO CONTINUE TO HIRE AND RETAIN SKILLED TECHNICAL AND SCIENTIFIC
PERSONNEL, THEN WE WILL HAVE TROUBLE DEVELOPING PRODUCTS.

     Our success depends largely upon the continued service of our management
and scientific staff and our ability to attract, retain and motivate highly
skilled scientific, management and marketing personnel. Potential employees with
an expertise in the field of biochemistry, regulatory affairs and/or clinical
development of new drug and biopharmaceutical manufacturing are not generally
available in the market and are difficult to attract and retain. We also face
significant competition for such personnel from other companies, research and
academic institutions, government and other organizations who have superior
funding and resources to be able to attract such personnel. The loss of key
personnel or our inability to hire and retain personnel who have technical and
scientific backgrounds could materially adversely affect our product development
efforts and our business.

OUR STOCK PRICE COULD BE VOLATILE, WHICH COULD CAUSE YOU TO LOSE PART OR ALL OF
YOUR INVESTMENT.

     The market price of our common stock, like that of the common stock of many
other development stage biotechnology companies, may be highly volatile. In
addition, the stock market has experienced extreme price and volume
fluctuations. This volatility has significantly affected the market prices of
securities of many biotechnology and pharmaceutical companies for reasons
frequently unrelated to or disproportionate to the operating performance of the
specific companies. These broad market fluctuations may adversely affect the
market price of our common stock.


                                      -9-

<PAGE>

                                LEGAL PROCEEDINGS

     As referenced in our quarterly report on Form 10-Q for the period ending
June 30, 1999 on July 17, 1998, we filed a complaint against Bristol-Myers
Squibb Corporation ("BMS") at the United States District Court for the District
of Massachusetts in Boston, Massachusetts seeking correction of inventorship of
certain United States patents which claim compositions and methods of use for
CTLA4 as well as unspecified monetary damages. A correction of inventorship
would result in the University of Michigan being designated as a co-assignee on
any corrected BMS patent. We would then have rights to such technology pursuant
to a 1992 License Agreement with the University of Michigan, a 1995 Asset
Acquisition Agreement with Genetics Institute, and other related agreements. On
July 13, 1999, the court dismissed the complaint without prejudice citing a lack
of legal standing of Repligen to bring such a complaint. We believe that the
court's finding on standing was in error. The court did not rule on the validity
of our inventorship claim. We continue to believe that the University of
Michigan is a rightful co-assignee of the aforesaid BMS patents and we intend to
continue to pursue the correction of inventorship. Our failure to obtain shared
ownership rights in the patents may restrict our ability to commercialize
CTLA4-Ig.

                                 USE OF PROCEEDS

     We will not receive any of the proceeds from the sale of our common stock
by the selling stockholders. See "Selling Stockholders" and "Plan of
Distribution". The principal purpose of this offering is to effect an orderly
disposition of the shares of our common stock being offered and sold from time
to time by the selling stockholders.


                                      -10-

<PAGE>


                              SELLING STOCKHOLDERS

     Unless otherwise noted below in the table, each person has sole voting and
investment power over the shares shown as beneficially owned except to the
extent authority is shared by spouses under applicable law and except as set
forth in the footnotes to the table. Beneficial ownership is determined in
accordance with the rules of the Securities and Exchange Commission. The
following are deemed to be beneficially owned and outstanding for purposes of
calculating the number of shares and the percentage beneficially owned by that
person or entity:

     -    shares of common stock issuable by Repligen to that person upon the
          exercise of options or warrants which may be exercised within 60 days
          after May 3, 2000.

However, these shares are not deemed to be beneficially owned and outstanding
for purposes of computing the percentage beneficially owned by any other person
or entity.

     For purposes of calculating the percentage beneficially owned, the number
of shares deemed outstanding after the offering includes:

     -    25,978,899 shares of common stock outstanding as of March 24, 2000;
          and

     -    the number of presently exercisable options and presently exercisable
          warrants held by that person.

The table below was prepared based upon information furnished to us by the
selling stockholders and lists the following:

     -    the number of shares of our common stock beneficially owned by the
          selling stockholders as of March 24, 2000 and before this offering;

     -    the maximum number of shares of our common stock that the selling
          stockholders may offer and sell pursuant to this prospectus;

     -    the number of shares owned by the selling stockholders after
          completion of the offering (assuming that the selling stockholders
          sell all of the shares offered pursuant to this prospectus); and

     -    the percentage (if one percent or more) of the class owned by the
          selling stockholder after completion of the offering.


<TABLE>
<CAPTION>

                                                                                                         SHARES BENEFICIALLY
                                                                                                       OWNED AFTER OFFERING (1)
                                                           SHARES                 SHARES             ----------------------------
                                                        BENEFICIALLY              OFFERED
                                                       OWNED PRIOR TO            PURSUANT TO
                   SELLING STOCKHOLDER                    OFFERING             THIS PROSPECTUS          NUMBER           PERCENT
- --------------------------------------------------      --------------         ---------------          ------           -------

<S>                                                    <C>                     <C>                     <C>             <C>
Alan Zverin                                                  2,898                   2,898                    0             *
Albert Fried & Company LLC                                 105,833                 105,833                    0             *
Alexander Pomper                                            10,000                  10,000                    0             *
Anthony G. Polak (2)                                         2,898                   2,898                    0             *
Anthony G. Polak "S" (3)                                     2,898                   2,898                    0             *

</TABLE>


                                  -11-

<PAGE>


<TABLE>
<CAPTION>

                                                                                                         SHARES BENEFICIALLY
                                                                                                       OWNED AFTER OFFERING (1)
                                                           SHARES                 SHARES             ----------------------------
                                                        BENEFICIALLY              OFFERED
                                                       OWNED PRIOR TO            PURSUANT TO
                   SELLING STOCKHOLDER                    OFFERING             THIS PROSPECTUS          NUMBER           PERCENT
- --------------------------------------------------      --------------         ---------------          ------           -------

<S>                                                    <C>                     <C>                     <C>             <C>
Anthony J. Gerace                                            8,035                   8,035                    0             *
Beck Family Partners, L.P.                                   5,797                   5,797                    0             *
Caduceus Capital II, L.P. (4)                               58,000                  58,000                    0             *
David Wilstein and Susan Wilstein Trustees of               17,390                  17,390                    0             *
     the Century Trust
Deephaven Opportunity Trading Fund L.P.                     34,782                  34,782                    0             *
Domaco Venture Capital Fund                                  2,898                   2,898                    0             *
Don Chaifetz                                                30,000                  15,000               15,000             *
Donald C. Carter                                            20,000                  20,000                    0             *
Douglas and Laurie Moore Trustees FBO the `89               12,000                  10,000                2,000             *
     Moore Family Trust dated 3/9/89
Evan Myrianthopoulos                                         6,000                   6,000                    0             *
Ezra S. Kazam                                               11,590                  11,590                    0             *
Frederick B. Polak                                           2,898                   2,898                    0             *
Gemini Domestic Fund II, L.P.                               88,000                  88,000                    0             *
Gemini Domestic Fund, L.P.                                   7,000                   7,000                    0             *
Goldman Sachs Strategic Technology Portfolio,               25,000                  25,000                    0             *
     L.P. (5)
John M. Hanley                                               5,000                   5,000                    0             *
John Nicholson (6)                                           5,000                   5,000                    0             *
John S. Gross                                                2,898                   2,898                    0             *
Jonathan Rothschild                                         13,000                   2,898               10,102             *
Joseph A. Galati                                             5,000                   5,000                    0             *
Joseph A. Natiello                                          13,200                  10,000                3,200             *
Joseph E. Edelman                                           23,188                  23,188                    0             *
Joseph Strassman and Barbara Strassman as                   30,000                  30,000                    0             *
     Tenants in Common
Joshua B. Stern                                             12,000                  12,000                    0             *
Kevin P. Newman                                              2,500                   2,500                    0             *
Linda Ann Hamilton (7)                                       5,797                   5,797                    0             *
M.S.B. Research                                             50,000                  50,000                    0             *
Margrit Polak                                                2,898                   2,898                    0             *
Mark C. Rogers, M.D.                                        12,000                  12,000                    0             *
Marc Florin                                                 21,167                  21,167                    0             *
Mark Mazzer                                                  3,000                   3,000                    0             *
Maureen McEnroe                                              3,000                   3,000                    0             *
Med-Tec Investors, LLC                                      10,000                  10,000                    0             *
Merlin BioMed International Ltd.                           155,000                 155,000                    0             *
</TABLE>


                                      -12-

<PAGE>


<TABLE>
<CAPTION>

                                                                                                         SHARES BENEFICIALLY
                                                                                                       OWNED AFTER OFFERING (1)
                                                           SHARES                 SHARES             ----------------------------
                                                        BENEFICIALLY              OFFERED
                                                       OWNED PRIOR TO            PURSUANT TO
                   SELLING STOCKHOLDER                    OFFERING             THIS PROSPECTUS          NUMBER           PERCENT
- --------------------------------------------------      --------------         ---------------          ------           -------

<S>                                                    <C>                     <C>                     <C>             <C>
Merlin BioMed New Issues, LLC                               17,000                  17,000                    0             *
Merlin BioMed, L.P.                                        100,000                 100,000                    0             *
Michael C. Miles                                            10,000                  10,000                    0             *
Michael H. Schwartz Profit Plan                             10,000                  10,000                    0             *
Michael M. Schultz                                          10,000                  10,000                    0             *
Mitchell Silber                                             25,000                  25,000                    0             *
Paul M. Millman                                              2,898                   2,898                    0             *
Perceptive Life Sciences L.P.                               23,188                  23,188                    0             *
Peter M. Kash (8)                                           11,590                  11,590                    0             *
Petrus Fund L.P.                                            30,000                  30,000                    0             *
Phoenix Fund Ltd.                                          115,000                 115,000                    0             *
Pimco Opportunity Fund                                     350,000                 347,826                2,174             *
Premero Investments Ltd.                                    11,591                  11,591                    0             *
PW Eucalyptus Fund LLC (9)                                 177,000                 177,000                    0             *
Quantum Partners, LDC                                      405,000                 405,000                    0             *
Richard Passarelli                                           5,000                   5,000                    0             *
RL Capital Partners (10)                                    17,391                  17,391                    0             *
Robert J. Capetola                                          13,000                  13,000                    0             *
Robert J. Conrads                                            3,000                   3,000                    0             *
Roger and Margaret Coleman                                   5,000                   5,000                    0             *
Ronald Lazar (11)                                            5,797                   5,797                    0             *
S. Edmond Farber                                             5,797                   5,797                    0             *
Sands Point Partners (12)                                   50,000                  50,000                    0             *
Sean C. Twomey                                               2,500                   2,500                    0             *
Southshore Capital Fund Limited                             57,971                  57,971                    0             *
St. John's Trust                                            34,782                  34,782                    0             *
Steven M. Oliveira                                          20,000                  20,000                    0             *
Stuart Weisbrod                                             50,000                  50,000                    0             *
The Gemini Master Fund                                      20,000                  20,000                    0             *
The William J. von Liebig                                   64,580                  40,580               20,000             *
     Foundation (13)
Trevor Colby                                                 9,855                   9,855                    0             *
Wayne Rothbaum                                              27,000                  27,000                    0             *
William W. Rider                                             5,000                   5,000                    0             *
</TABLE>


                                      -13-

<PAGE>


<TABLE>
<CAPTION>

                                                                                                         SHARES BENEFICIALLY
                                                                                                       OWNED AFTER OFFERING (1)
                                                           SHARES                 SHARES             ----------------------------
                                                        BENEFICIALLY              OFFERED
                                                       OWNED PRIOR TO            PURSUANT TO
                   SELLING STOCKHOLDER                    OFFERING             THIS PROSPECTUS          NUMBER           PERCENT
- --------------------------------------------------      --------------         ---------------          ------           -------

<S>                                                    <C>                     <C>                     <C>             <C>
Winchester Global Trust Company Limited as                 129,000                 129,000                    0             *
     Trustee for Caduceus Capital Trust (13)
Wolfe F. Model                                               2,898                   2,898                    0             *
Paramount Capital, Inc. (14)                             4,689,646                 129,946            4,559,700           17.4
</TABLE>

- -------------------------------------
*Represents less than 1% of the outstanding shares.

(1)      Assumes that the selling stockholders will sell all of the shares
         registered hereunder. The stockholders may sell all or part of their
         shares pursuant to this prospectus.

(2)      Does not include 10,000 shares beneficially owned by Anthony G. Polak
         over which Anthony G. Polak has sole voting and investment power.

(3)      Does not include 10,000 shares beneficially held in the Anthony G.
         Polak "S" account over which Anthony G. Polak has sole voting and
         investment power.

(4)      The shares beneficially owned do not include shares offered pursuant to
         this prospectus by PW Eucalyptus Fund LLC ("PW Eucalyptus") or
         Winchester Global Trust Company Limited as Trustee for Caduceus Capital
         Trust ("Caduceus Capital Trust"). Orbimed Advisors LLC has sole
         investment power over the shares held by each of Caduceus Capital II,
         L.P. ("Caduceus Capital"), PW Eucalyptus and Caduceus Capital Trust.
         Caduceus Capital disclaims beneficial ownership of the shares held by
         PW Eucalyptus and Caduceus Capital Trust.

(5)      Goldman Sachs Strategic Technology Portfolio, L.P. and its investment
         manager, Commodities Corporation LLC, are affiliates of Goldman Sachs &
         Co. The shares beneficially owned do not include shares which may be
         held by Goldman Sachs & Co. and its affiliates other than Goldman Sachs
         Strategic Technology Portfolio, L.P. and Commodities Corporation LLC.

(6)      The shares beneficially owned do not include shares offered pursuant to
         this prospectus by Sands Point Partners of which Mr. Nicholson is a
         general partner. Mr. Nicholson has shared voting and investment power
         over the shares beneficially owned by Sands Point Partners. Mr.
         Nicholson may be deemed to beneficially own the shares of Sands Point
         Partners.

(7)      Does not include shares beneficially owned by The William J. von
         Liebig Foundation of which Ms. Hamilton is a director.


                                      -14-


<PAGE>


(8)      Mr. Kash is the Senior Managing Director of Paramount Capital Inc.
         and a member of the board of directors of Paramount Capital Asset
         Management, Inc. The shares beneficially owned do not include shares
         offered pursuant to this Prospectus by Paramount Capital, Inc. and
         Mr. Kash disclaims beneficial ownership of such shares except to the
         extent of his pecuniary interest therein, if any.

(9)      The shares beneficially owned do not include shares offered pursuant to
         this prospectus by Caduceus Capital and Caduceus Capital Trust. Orbimed
         Advisors LLC has sole investment power over the shares held by each of
         PW Eucalyptus, Caduceus Capital and Caduceus Capital Trust. PW
         Eucalyptus disclaims beneficial ownership of the shares held by
         Caduceus Capital and Caduceus Capital Trust.

(10)     The shares beneficially owned do not include shares offered pursuant to
         this prospectus by Ronald Lazar. Mr. Lazar is a general partner of RL
         Capital Partners.

(11)     The shares beneficially owned do not include shares offered pursuant to
         this prospectus by RL Capital Partners. Mr. Lazar is a general partner
         of RL Limited Partners.

(12)     The shares beneficially owned do not include shares offered pursuant to
         this prospectus by John Nicholson. Mr. Nicholson is a general partner
         of Sands Point Partners. Mr. Nicholson has shared voting and investment
         power over the shares beneficially owned by Sands Point Partners. Mr.
         Nicholson may be deemed to beneficially own the shares of Sands Point
         Partners.

(13)     Does not include shares beneficially owned by Linda Ann Hamilton. Ms.
         Hamilton is a director of The William J. von Liebig Foundation.

(14)     The shares beneficially owned do not include shares offered pursuant to
         this prospectus by Caduceus Capital and PW Eucalyptus. Orbimed Advisors
         LLC has sole investment power over the shares held by each of Caduceus
         Capital Trust, Caduceus Capital and PW Eucalyptus. Caduceus Capital
         Trust disclaims beneficial ownership of the shares held by Caduceus
         Capital and PW Eucalyptus.

(15)     The shares beneficially owned consist of 129,946 shares of common stock
         which may be acquired upon the exercise of a warrant which may be
         exercised beginning on September 9, 2000 and offered for sale pursuant
         to this prospectus, 3,108,671 shares of common stock owned by The Aries
         Master Fund, a Cayman Islands exempted company (the "Master Fund"),
         1,223,192 shares of common stock owned by Aries Domestic Fund, L.P.
         (the "ADF") and 195,087 shares of common stock owned by Aries Domestic
         Fund II, L.P. (the "ADF II" and together with the Master Fund and the
         ADF, the "Aries Funds") and 32,750 shares of common stock which may be
         acquired pursuant to presently exercisable warrants owned by Dr.
         Lindsay Rosenwald. Paramount Capital Asset Management, Inc. ("PCAM")
         serves as the investment manager to the Master Fund and the general
         partner of each of ADF and ADF II. Lindsay A. Rosenwald, M.D. is the
         chairman and sole stockholder of PCAM. PCAM and Dr. Rosenwald disclaim
         beneficial ownership of the securities held by ADF, the Master Fund
         and ADF II, except to the extent of their pecuniary interest therein,
         if any. Does not include shares of our common stock which may be owned
         or which may be acquired pursuant to presently exercisable warrants
         owned by certain other employees of PCAM and its affiliates. We issued
         the warrants to Dr. Rosenwald and such other employees of PCAM and its
         affiliates in connection with Paramount's execution of a Financial
         Advisory Agreement dated July 15, 1999 between Paramount and Repligen.
         Paramount disclaims beneficial


                                      -15-

<PAGE>

         ownership of such shares of common stock except to the extent of its
         pecuniary interest therein, if any.
- -------------------------------------


     We are filing this registration statement to register for public sale the
shares of common stock currently held by the selling stockholders, or which may
be acquired upon exercise of the warrants held by the them.

     The selling stockholders have represented to us that they acquired the
shares as principal for their own accounts for investment and not with a view
to, or for sale in connection with, any distribution of the shares in
contravention of the Securities Act or any other applicable securities
legislation. In recognition of the fact, however, that the selling stockholders
may want to be able to sell the shares when they consider it appropriate, in
connection with the stock purchase agreement, we agreed to file the registration
statement with the Commission to effect the registration of the resale of the
shares under the Securities Act and to keep the registration statement effective
until the earliest of (i) the date on which the selling stockholders may sell
all shares then held by them without restriction under Rule 144(k) of the
Securities Act, (ii) such time as all shares purchased by the selling
stockholders hereunder have been sold pursuant to this registration statement or
(iii) March 9, 2002.

     On March 9, 2000, we sold an aggregate of 2,598,927 shares of our common
stock to investors at $8.625 per share for an aggregate consideration of $22.4
million in a private placement pursuant to a stock purchase agreement by and
among the investors and us. These investors are offering for sale these
2,598,927 shares of our common stock pursuant to this prospectus. Paramount
acted as the placement agent for the transaction and we paid Paramount
approximately $1.57 million for its services plus Paramount's related
transactional expenses, and issued to Paramount warrants to purchase up to
129,946 shares of our common stock at $9.49 per share. We engaged Paramount to
act as our placement agent for that transaction pursuant to a finder's agreement
and we terminated the financial advisory agreement (described in the paragraph
immediately below) with Paramount for an additional payment by Repligen to
Paramount of $200,000 in cash. Pursuant to this prospectus, Paramount is
offering for sale the 129,946 shares of our common stock upon exercise of the
warrants issued to it in connection with the private placement transaction.

     Pursuant to a financial advisory agreement with Paramount Capital, Inc. its
affiliates, related entities and designees dated as of July 15, 1999, Paramount
agreed to provide certain financial advisory and/or investment banking services
to us in exchange for consideration, including $100,000 in cash upon execution
of the agreement, and the issuance of a common stock purchase warrant
exercisable for 100,000 shares of Repligen's common stock at $2.75 per share.
The financial advisory agreement also provided for the payment of additional
fees to Paramount upon the consummation of certain transactions for which
Paramount acted as our financial advisor. We have terminated this financial
advisory agreement as set forth in the paragraph immediately above.

     On May 14, 1999, the ADF and the Master Fund acquired an aggregate of
50,000 shares of our common stock at $2.50 per share in connection with a
private placement transaction in


                                      -16-

<PAGE>

which we sold an aggregate of 3,600,000 shares of our common stock at $2.50 per
share. As of December 31, 1999, Dr. Rosenwald and PCAM may be deemed to
beneficially own approximately seventeen percent (17%) of the outstanding our
common stock. Paramount disclaims beneficial ownership of the shares of common
stock owned by Dr. Rosenwald, PCAM and/or the Aries Funds.

     Except as noted above and based on representations by the selling
stockholders, to the best of our knowledge, no other selling stockholders had a
material relationship with us or any of our affiliates within the three-year
period ending on the date of this prospectus.



                              PLAN OF DISTRIBUTION

     The shares of our common stock covered by this prospectus may be sold by
the selling stockholders from time to time for their own account or by pledgees,
donees, transferees, designees, or other successors in interest. Without
limiting the generality of the preceding sentence, the warrant which was issued
to Paramount Capital, Inc. to acquire up to 129,946 shares of our common stock
at $9.49 per share in connection with the March 2000 private placement may at
any time be transferred to employees and or designees of Paramount or its
affiliates for no additional consideration. This prospectus is expressly
intended to cover and allow any shares of our common stock acquired upon
exercise of the warrant originally issued to Paramount to be sold by such
transferees pursuant to this prospectus. We will pay the expenses incurred in
connection with the registration of the shares of our common stock sold
hereunder, except that the selling stockholders will pay or assume brokerage
commissions and similar charges, the legal fees and expenses of counsel for the
selling stockholders and any stock transfer taxes or other expenses incurred in
connection with the sale of the shares of our common stock. We will not receive
any of the proceeds from the resale of the shares of common stock by the selling
stockholders.

     The distribution of the shares of our common stock by the selling
stockholders is not subject to any underwriting agreement. The shares of our
common stock offered by the selling stockholders may be sold from time to time
in transactions on the Nasdaq National Market, in negotiated transactions,
through the writing of options on the shares of our common stock, or a
combination of such methods of sale, at fixed prices that may be changed, at
market prices prevailing at the time of sale, at prices relating to such
prevailing market prices or at negotiated prices. Any broker-dealers that
participate with selling stockholders in the distribution of shares of our
common stock may be deemed to be underwriters and any commissions received by
them and any profit on the resale of shares of our common stock placed by them
might be deemed to be underwriting discounts and commissions within the meaning
of the Securities Act, in connection with such sales.

     Any shares covered by the prospectus that qualify for sale pursuant to Rule
144 under the Securities Act may be sold under Rule 144 rather than pursuant to
this prospectus.

     Since the selling stockholders are not restricted as to the price or prices
at which they may sell their shares of our common stock, sales of such shares of
our common stock at less than the market prices may depress the market price of
the our common stock.


                                      -17-

<PAGE>

     EquiServe, 150 Royall Street, Canton, MA 02021, is the transfer agent for
the shares of common stock of Repligen.

                                  LEGAL MATTERS

     The validity of the shares of our common stock offered hereby will be
passed upon for us by Testa, Hurwitz & Thibeault, LLP, Boston, Massachusetts.

                                     EXPERTS

     The financial statements incorporated by reference in this prospectus and
elsewhere in the registration statement to the extent and for the periods
indicated in their reports have been audited by Arthur Andersen LLP, independent
public accountants, as indicated in their reports with respect thereto, and are
included herein in reliance upon the authority of said firm as experts in
auditing and accounting.

                       WHERE YOU CAN FIND MORE INFORMATION

     Repligen files annual, quarterly and special reports, proxy statements and
other information with the SEC. You may read and copy any document we file with
the SEC at the SEC's public reference room at 450 Fifth Street, N.W.,
Washington, D.C. 20549. Please call the SEC at 1-800-SEC-0330 for further
information on operation of the public reference room. Our SEC filings are also
available to the public from the SEC's website at "http://www.sec.gov." Our
website is located at "http://www.repligen.com." Information contained on our
website is not part of this prospectus.

     The SEC allows Repligen to "incorporate by reference" the information we
file with them, which means that we can disclose important information to you by
referring you to those documents. The information incorporated by reference is
considered to be part of this prospectus, and information that we file later
with the SEC will automatically update and supersede this information. Repligen
incorporates by reference the documents listed below and any future filings we
will make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Exchange
Act (File No. 000-14656):

1.   Annual report on Form 10-K for the year ended March 31, 1999;

2.   Repligen's proxy statement, filed on July 29, 1999, for the 1999 annual
     meeting of shareholders;

3.   Quarterly reports on Form 10-Q for the quarters ended December 31, 1999,
     September 30, 1999 and June 30, 1999;

4.   Current reports on Form 8-K filed March 21, 2000, October 6, 1999, April
     24, 1999, as amended by Form 8-K/A filed June 15, 1999 and current report
     on Form 8-K filed May 17, 1999; and


                                      -18-

<PAGE>

5.   The "Description of Registrant's Securities to be Registered" contained in
     Repligen's registration statement filed on Form 8-A, dated May 28, 1986.

     You may request a copy of these filings, at no cost, by writing or
telephoning our Chief Financial Officer at the following address: Repligen
Corporation, 117 Fourth Avenue Needham, MA 02494 (781) 449-9560.

     This prospectus is part of a registration statement we filed with the SEC.
You should rely only on the information or representations provided in this
prospectus. We have authorized no one to provide you with different information.
We are not making an offer of these securities in any state where the offer is
not permitted. You should not assume that the information in this prospectus is
accurate as of any date other than the date on the front of the document.


                                      -19-


<PAGE>


                                     PART II

                     INFORMATION NOT REQUIRED IN PROSPECTUS

ITEM 14.  OTHER EXPENSES OF ISSUANCE AND DISTRIBUTION.

     The following table sets forth an estimate (other than with respect to the
Registration Fee) of the expenses expected to be incurred in connection with the
issuance and distribution of the securities being registered, other than
underwriting discounts and commissions:

Registration Fee -- Securities and Exchange Commission............$     4,504.00
Blue Sky Fees and Expenses........................................$     1,000.00
Accounting Fees and Expenses......................................$     1,000.00
Legal Fees and Expenses...........................................$    15,000.00
Transfer Agent Fees and Expenses..................................$     3,000.00
Miscellaneous.....................................................$    10,496.00
                                                                  --------------
         TOTAL....................................................$    35,000.00
                                                                  ==============

         Repligen will bear all expenses shown above.

ITEM 15.  INDEMNIFICATION OF DIRECTORS AND OFFICERS.

     The Delaware General Corporation Law, Article Seventh of Repligen's
Restated Certificate of Incorporation, as amended, and Article V of Repligen's
By-laws provide for indemnification of Repligen's directors and officers for
liabilities and expenses that they may incur in such capacities. In general,
directors and officers are indemnified with respect to actions taken in good
faith in a manner reasonably believed to be in, or not opposed to, the best
interests of Repligen, and with respect to any criminal action or proceeding,
actions that the indemnitee had no reasonable cause to believe were unlawful.

     Repligen maintains directors and officers liability insurance for the
benefit of its directors and certain of its officers.

ITEM 16. EXHIBITS.

     The following exhibits, required by Item 601 of Regulation S-K, are filed
as a part of this Registration Statement. Exhibit numbers, where applicable, in
the left column correspond to those of Item 601 of Regulation S-K.

EXHIBIT NO.   ITEM AND REFERENCE
- ----------    ------------------------------------------------------------------

4.1       --  Stock Purchase Agreement dated as of March 7, 2000, by and among
              Repligen Corporation and the Investors listed on Schedule I
              thereto (filed as Exhibit 4.1 to the Company Current Report on
              Form 8-K filed March 21, 2000 and incorporated herein by
              reference)


                                      II-1
<PAGE>



4.2       --  Finders Agreement by and between Repligen Corporation and
              Paramount Capital, Inc. dated as of March 2, 2000 (filed as
              Exhibit 4.2 to the Company's Current Report on Form 8-K filed
              March 21, 2000 and incorporated herein by reference)

4.3       --  Form of Common Stock Purchase Warrant (filed herewith)

5         --  Legal Opinion of Testa, Hurwitz & Thibeault, LLP (filed
              herewith)

10.1      --  Financial Advisory Agreement with Paramount Capital, Inc.
              (filed as Exhibit 10.1 to the Company's Quarterly Report on Form
              10Q for the period ended September 30, 1999 and incorporated
              herein by reference)

10.2      --  Letter Agreement terminating July 1, 1999 Financial Advisory
              Agreement, Agreement to pay Termination Fee and Mutual Release by
              and between Repligen Corporation and Paramount Capital, Inc. dated
              as of March 8, 2000 (filed herewith)

23.1      --  Consent of Arthur Andersen LLP (filed herewith)

23.2      --  Consent of Testa, Hurwitz & Thibeault, LLP (included in Exhibit 5)

24        --  Power of Attorney (included on signature pages)

ITEM 17. UNDERTAKINGS.

The undersigned registrant hereby undertakes:

     (1) To file, during any period in which offers or sales are being made, a
post-effective amendment to this registration statement:

         (i) To include any prospectus required by Section 10(a)(3) of the
 Securities Act of 1933;

         (ii) To reflect in the prospectus any facts or events arising after the
effective date of the registration statement (or the most recent post-effective
amendment thereof) which, individually or in the aggregate, represents a
fundamental change in the information set forth in the registration statement.
Notwithstanding the foregoing, any increase or decrease in volume of securities
offered (if the total dollar value of securities offered would not exceed that
which was registered) and any deviation from the low or high and of the
estimated maximum offering range may be reflected in the form of prospectus
filed with the Commission pursuant to Rule 424(b) if, in the aggregate, the
changes in volume and price represent no more than 20 percent change in the
maximum aggregate offering price set forth in the "Calculation of Registration
Fee" table in the effective registration statement;

         (iii) To include any material information with respect to the plan of
distribution not previously disclosed in the registration statement or any
material change to such information in the registration statement.


                                      II-2

<PAGE>

     (2) That, for the purpose of determining any liability under the Securities
Act of 1933, each such post-effective amendment shall be deemed to be a new
registration statement relating to the securities offered therein, and the
offering of such securities at that time shall be deemed to be the initial BONA
FIDE offering thereof.

     (3) To remove from registration by means of a post-effective amendment any
of the securities being registered which remain unsold at the termination of the
offering.

     The undersigned registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act of 1933, each filing of the
registrant's annual report pursuant to Section 13(a) or 15(d) of the Securities
Exchange Act of 1934 (and, where applicable, each filing of an employee benefit
plan's annual report pursuant to Section 15(d) of the Securities Exchange Act of
1934) that is incorporated by reference in the registration statement shall be
deemed to be a new registration statement relating to the securities offered
therein, and the offering of such securities at that time shall be deemed to be
the initial BONA FIDE offering thereof.

     Insofar as indemnification for liabilities arising under the Securities Act
of 1933 may be permitted to directors, officers and controlling persons of the
registrant pursuant to provisions described in Item 15 above, or otherwise, the
registrant has been advised that in the opinion of the Securities and Exchange
Commission such indemnification is against public policy as expressed in the
Securities Act of 1933 and is, therefore, unenforceable. In the event that a
claim for indemnification against such liabilities (other than the payment by
the registrant of expenses incurred or paid by a director, officer or
controlling person of the registrant in the successful defense of any action,
suit or proceeding) is asserted by such director, officer or controlling person
in connection with the securities being registered, the registrant will, unless
in the opinion of its counsel the matter has been settled by controlling
precedent, submit to a court of appropriate jurisdiction the question whether
such indemnification by it is against public policy as expressed in the
Securities Act of 1933 and will be governed by the final adjudication of such
issue.


                                      II-3


<PAGE>


                                   SIGNATURES

     Pursuant to the requirements of the Securities Act of 1933, the registrant
certifies that it has reasonable grounds to believe that it meets all of the
requirements for filing on Form S-3 and has duly caused this Registration
Statement on Form S-3 to be signed on its behalf by the undersigned, thereunto
duly authorized in the Town of Needham, Commonwealth of Massachusetts on May 4,
2000.

                                               Repligen Corporation


                                               By: /s/ Walter C. Herlihy
                                                   -----------------------------
                                                   Walter C. Herlihy
                                                   President and Chief Executive
                                                     Officer


                                POWER OF ATTORNEY

     Each person whose signature appears below on this Registration Statement
hereby constitutes and appoints Walter C. Herlihy with full power to act as his
true and lawful attorney-in-fact and agent, with full power of substitution and
resubstitution, for him and in his name, place and stead, in any and all
capacities (until revoked in writing) to sign any and all amendments (including
post-effective amendments and amendments thereto) to this Registration Statement
on Form S-3 of Repligen Corporation, and to file the same, with all exhibits
thereto, and other documents in connection therewith, with the Securities and
Exchange Commission, granting unto said attorney-in-fact and agents full power
and authority to do and perform each and every act and thing requisite and
necessary fully to all intents and purposes as he might or could do in person
thereby ratifying and confirming all that said attorney-in-fact and agents or
any of them, or their or his substitute or substitutes, may lawfully do or cause
to be done by virtue hereof.

     Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement on Form S-3 has been signed below by the following
persons on behalf of the Registrant and in the capacities and on the dates
indicated.


<TABLE>
<CAPTION>

NAME                                            CAPACITY                                   DATE

<S>                                            <C>                                       <C>
/s/ Walter C. Herlihy
- -------------------------------                 President and Chief Executive Officer,     May 4, 2000
Walter C. Herlihy                               Chief Financial Officer and Director
                                                (principal executive, financial and
                                                 accounting officer)

/s/ Alexander Rich                              Co-Chairman of the Board of Directors      May 4, 2000
- -------------------------------
Alexander Rich, M.D.


                                      II-4

<PAGE>

/s/ Paul Schimmel                               Co-Chairman of the Board of Directors      May 4, 2000
- -------------------------------
Paul Schimmel, Ph.D.

/s/ Robert J. Hennessey                         Director                                   May 4, 2000
- -------------------------------
Robert J. Hennessey

/s/ G. William Miller                           Director                                   May 4, 2000
- -------------------------------
G. William Miller
</TABLE>


                                      II-5


<PAGE>


                                INDEX TO EXHIBITS

EXHIBIT NO.   ITEM AND REFERENCE
- ----------    -----------------------------------------------------------------

4.1        -- Stock Purchase Agreement dated as of March 7, 2000, by and
              among Repligen Corporation and the Investors listed on Schedule I
              thereto (filed as Exhibit 4.1 to the Company Current Report on
              Form 8-K filed March 21, 2000 and incorporated herein by
              reference)

4.2        -- Finders Agreement by and between Repligen Corporation and
              Paramount Capital, Inc. dated as of March 2, 2000 (filed as
              Exhibit 4.2 to the Company's Current Report on Form 8-K filed
              March 21, 2000 and incorporated herein by reference)

4.3        -- Form of Common Stock Purchase Warrant (filed herewith)

5          -- Legal Opinion of Testa, Hurwitz & Thibeault, LLP (filed
              herewith)

10.1       -- Financial Advisory Agreement with Paramount Capital, Inc.
              (filed as Exhibit 10.1 to the Company's Quarterly Report on Form
              10Q for the period ended September 30, 1999 and incorporated
              herein by reference)

10.2       -- Letter Agreement terminating July 1, 1999 Financial Advisory
              Agreement, Agreement to pay Termination Fee and Mutual Release by
              and between Repligen Corporation and Paramount Capital, Inc. dated
              as of March 8, 2000 (filed herewith)

23.1       -- Consent of Arthur Andersen LLP (filed herewith)

23.2       -- Consent of Testa, Hurwitz & Thibeault, LLP (included in Exhibit 5)

24         -- Power of Attorney (included on signature pages)



<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:


_____________________________




<PAGE>

                                  [LETTERHEAD]

                                                                     May 4, 2000


Repligen Corporation
117 Fourth Avenue
Needham, Massachusetts 02194

         Re: S-3 REGISTRATION STATEMENT

Ladies and Gentlemen:

         We are counsel to Repligen Corporation, a Delaware corporation (the
"Company"), and have represented the Company in connection with the preparation
and filing of the Company's Form S-3 Registration Statement (the "Registration
Statement"), covering the sale to the public of up to 2,598,927 shares of the
Company's common stock (the "Common Stock"), $.01 par value per share (the
"Shares"), and up to 129,946 shares of the Company's Common Stock (the "Warrant
Shares") which a selling stockholder may acquire upon the exercise of a warrant
described in the Registration Statement, all of which may be sold by certain
selling stockholders of the Company as set forth in the Registration Statement.

         We have reviewed the corporate proceedings taken by the Board of
Directors of the Company with respect to the authorization and issuance of the
Shares and the Warrant Shares. We have also examined and relied upon originals
or copies, certified or otherwise authenticated to our satisfaction, of all
corporate records, documents, agreements or other instruments of the Company and
have made all investigations of law and have discussed with the Company's
officers all questions of fact that we have deemed necessary or appropriate.

         Based upon and subject to the foregoing, we are of the opinion that the
Shares are legally issued, fully paid and non-assessable and the Warrant Shares,
when issued upon exercise of the warrant in accordance with the terms therein,
will be legally issued, fully paid and non-assessable.

         We hereby consent to the filing of this opinion as Exhibit 5 to the
Registration Statement and to the reference to our firm in the Prospectus
contained in the Registration Statement under the caption "Legal Matters."

                                           Very truly yours,

                                           /s/ TESTA, HURWITZ & THIBEAULT, LLP

                                           TESTA, HURWITZ & THIBEAULT, LLP



<PAGE>

                              Repligen Corporation
                                117 Fourth Avenue
                                Needham, MA 02494


                                                              March 8, 2000


Paramount Capital, Inc.
787 Seventh Avenue
48th Floor
New York, NY 10019
ATTN: David Tanen

         RE: Termination of July 1, 1999 Financial Advisory Agreement,
             AGREEMENT TO PAY TERMINATION FEE, AND MUTUAL RELEASE

Dear Mr. Tanen:

         This letter (1) memorializes the termination of the Financial Advisory
Agreement by and between Repligen Corporation (the "COMPANY") and Paramount
Capital, Inc., its affiliates, related entities and designees ("PARAMOUNT"),
dated July 1, 1999 (the "ADVISORY AGREEMENT"); (2) memorializes the Company's
conditional agreement to pay to Paramount a termination fee as set forth in
Section 2 below if, and only if, this letter agreement is executed and returned
by Paramount no later than March 8, 2000 and the Company closes the transactions
contemplated by the Stock Purchase Agreement dated as of March 7, 2000 by and
among the Company and the parties thereto (the "Stock Purchase Agreement"); and
(3) memorializes the general mutual release by and between Paramount and the
Company.

         1.       TERMINATION OF THE ADVISORY AGREEMENT.

         Notwithstanding the language in the Advisory Agreement, the parties
hereby agree to immediately and completely terminate the Advisory Agreement and
all of their rights and obligations thereunder.

         2.       ADVISORY AGREEMENT TO PAY TERMINATION PAYMENT.

         In consideration for the termination of the Advisory Agreement and the
execution of this letter agreement, the Company promises to pay Paramount
$200,000.00 (the "TERMINATION PAYMENT") by wire transfer or by Company check
within one (1) business day after the closing of the transactions contemplated
by the Stock Purchase Agreement. The parties acknowledge and agree that the
Termination Payment shall be paid in lieu of any other payments which may be due
in respect of the Advisory Agreement or otherwise under the Advisory Agreement.
The parties further agree that the above promise to pay the Termination Payment
constitutes consideration for this letter agreement, and is not reflective of
the value of the Advisory Agreement nor of the validity of any claim for payment
under the Advisory Agreement.

<PAGE>

Paramount Capital, Inc.
March 8, 2000
Page 2

         3.       CONDITIONAL MUTUAL RELEASE.

         Paramount and the Company, in exchange for valuable consideration the
receipt of which is hereby acknowledged, for themselves and for any past,
present, and future employees, officers, directors, agents, attorneys,
shareholders, partners, representatives, affiliates, predecessors, successors,
heirs, executors, administrators, and assigns (together the "RELEASORS"), hereby
mutually, absolutely, unconditionally, and completely release, remise, and
forever discharge each other, as well as their respective past, present, and
future employees, officers, directors, agents, attorneys, shareholders,
partners, representatives, affiliates, predecessors, successors, heirs,
executors, administrators, and assigns (together the "RELEASEES") of and from
all debts, demands, actions, causes of action, suits, accounts, convenants,
contracts, agreements, torts, damages, and any and all claims, demands, and
liabilities whatsoever, of every name and nature, both in law and in equity,
whether liquidated or unliquidated, past, present or future, known or unknown,
foreseen or unforeseen, which the Releasors now have or have ever had against
the Releasees, whether asserted or could have been asserted, or whether related
to any claims which were asserted or could have been asserted, by way of claim,
counterclaim, or otherwise, regarding amounts due and owing for work performed
or any payment or other consideration owed or payable in connection with the
Advisory Agreement. This Conditional Mutual Release shall be deemed effective
upon the payment by the Company to Paramount of the Termination Payment.

         4.       MISCELLANEOUS.

         (a) This agreement is the entire agreement among Paramount and the
Company with respect to the subject matter hereof and supercedes all prior and
contemporaneous oral and written agreements and discussions.

         (b) This agreement may be executed in counterparts and shall constitute
one agreement binding on Paramount and the Company, notwithstanding that they
did not sign the same original or counterpart.

         (c) This agreement is binding upon and shall inure to the benefit of
Paramount and the Company, their respective agents, employees, representatives,
officers, directors, divisions, subsidiaries, affiliates, assignes, heirs and
successors-in-interest.

         (d) If any provision of this agreement is held invalid, the invalidity
shall not affect the other provisions which can be given effect without the
invalid provisions and to this end the provisions of this agreement are declared
severable.

                                      * * *

<PAGE>

Paramount Capital, Inc.
March 8, 2000
Page 3


If the foregoing is acceptable to Paramount, please cause this letter to be
executed and returned to me as evidence of your agreement to the terms herein as
of the date set forth above. Thank you for your attention to this matter.



                                           Very truly yours,

                                       /s/ Walter C. Herlihy
                                           -------------------------------------
                                           Walter C. Herlihy
                                           President and Chief Executive Officer


PARAMOUNT CAPITAL, INC.

/s/ Lindsay A. Rosenwald, M.D.
- --------------------------------------------
Name: Lindsay A. Rosenwald, M.D.

Title: Chairman



<PAGE>


Exhibit 23.1


                    CONSENT OF INDEPENDENT PUBLIC ACCOUNTANTS


     As independent public accountants, we hereby consent to the incorporation
by reference in this registration statement of our report dated May 14, 1999
included in Repligen Corporation's form 10-K for the year ended March 31, 1999
and to all references to our Firm included in this registration statement.


                                                      /s/ Arthur Andersen LLP

                                                          ARTHUR ANDERSEN LLP


Boston, Massachusetts
May 2, 2000




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