AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON , 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)
                          -----------------------------


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


                                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

                         CALCULATION OF REGISTRATION FEE


- ------------------------------------------------------------------------------------------------------------------------------------
  TITLE OF EACH CLASS OF SECURITIES TO BE   AMOUNT TO BE      PROPOSED MAXIMUM          PROPOSED MAXIMUM             AMOUNT OF
              REGISTERED                     REGISTERED    OFFERING PRICE PER SHARE*  AGGREGATE OFFERING PRICE*    REGISTRATION FEE*
- ------------------------------------------------------------------------------------------------------------------------------------
                                                                                                          
Common Stock, $0.01 par value per share
(issuable upon exercise of the Exchange      1,653,250            $8.00                   $13,226,000                 $3,491.66
Warrants)
- ------------------------------------------------------------------------------------------------------------------------------------
Common Stock, $0.01 par value per share
(issuable upon exercise of the Modified        353,800           $14.00                   $4,953,200                  $1,307.64
Original Warrants)
- ------------------------------------------------------------------------------------------------------------------------------------


*  Pursuant to Rule 457(g) under the Securities Act of 1933, the proposed
   maximum offering price per share, the proposed maximum aggregate offering
   price and the amount of the registration fee with respect to shares to be
   offered pursuant to Exchange Warrants and the Modified Original Warrants have
   been computed in accordance with clause (1) thereof.

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.
================================================================================


[begin red herring]

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.

[end red herring]

                  SUBJECT TO COMPLETION, DATED January 28, 2000

                              REPLIGEN CORPORATION

                                2,007,050 SHARES

                                  COMMON STOCK

     This Prospectus relates to 2,007,050 shares of common stock, $.01 par
value per share, of Repligen Corporation issuable upon exercise of warrants
previously issued by Repligen. Repligen's common stock is traded on the
Nasdaq National Market under the symbol "RGEN." The last reported sale price
of the common stock of Repligen on the Nasdaq National Market on January 26,
2000 was $5.00 per share.

     Repligen is offering 2,007,050 shares of its common stock issuable upon the
exercise of certain warrants originally issued by it in February 1992 in a
financing transaction to purchasers of units and to the sales agent for the
units and the agent's affiliates. Each unit consisted of a Class A limited
partnership interest in Repligen Clinical Partners, L.P. and a warrant to
purchase 2,900 shares of common stock of Repligen. Of the warrants originally
issued as part of the units, certain warrants were exchanged in March 1994 and
later modified in March 1995 to reduce the exercise price of such warrants (the
"Exchange Warrants"), and the remaining warrants which were not exchanged in
March 1994 were later modified in March 1995 to reduce the exercise price of
such warrants (the "Modified Original Warrants"). The terms and conditions of
the offer and sale of the shares issuable upon exercise of the warrants,
including the price per share, are governed by the provisions of the warrants as
described herein under the caption "Description of Warrants." Repligen will
receive all proceeds from the exercise of the warrants.



- -----------------------------------------------------------------------------------------------------------
                    Shares of common
                     stock issuable   Per Share      Aggregate    Underwriting                  Net
                    upon exercise of  Price to      Proceeds to   Discounts and    Other      Proceeds to
                      warrants         Public         Company      Commissions    Expenses     Company
- -----------------------------------------------------------------------------------------------------------

                                                                            
Modified Original     353,800         $9.00-        $3,184,200-       -0-         $30,000     $3,154,200-
Warrants(1)                           $14.00        $4,953,200                                $4,923,200
- -----------------------------------------------------------------------------------------------------------

Exchange                              $2.50/$3.50-  $5,216,289-       -0-         $30,000     $5,186,289-
Warrants(2)         1,653,250         $8.00         $13,226,000                               $13,196,000
- -----------------------------------------------------------------------------------------------------------


     (1)The Modified Original Warrants are exercisable at $9.00 per share until
the date that is 90 days after Repligen notifies the holder of the Modified
Original Warrants that the Nasdaq National Market closing price per share of the
common stock of Repligen equaled or exceeded $18.00 per share for any 20 out of
30 consecutive trading days, at which time the Modified Original Warrants will
become exercisable at $14.00 per share.

     (2)The exercise price of the first 1,000 shares issuable upon exercise of
the Exchange Warrants is $2.50, and the remaining 1,900 shares issuable upon
exercise of the Exchange Warrants are exercisable at $3.50. The exercise price
for all of the shares subject to the Exchange Warrants will increase to $8.00
per share on the date 90 days after Repligen notifies warrantholders that the
Nasdaq National Market closing price per share of the common stock equaled or
exceeded $12.00 per share for 20 out of 30 consecutive trading days.

                  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 January ___, 2000.


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......................................    6
Use of Proceeds........................................    6
Dilution...............................................    6
Description of the Warrants............................    7
Legal Matters..........................................   11
Experts................................................   11
Where You Can Find More Information....................   11


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


                                    REPLIGEN

     We develop new drugs for neurological disease, 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, Repligen licensed 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 which it intends to submit
to the FDA in 2000. Under terms of the agreement, Repligen made an upfront
payment upon execution of the agreement and, if the FDA approves the New Drug
Applications, the agreement obligates Repligen 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-Meyers Squibb Corporation relating to the use and manufacturing of
CTLA4-Ig. We believe that through one of our licenses we have rights

                                      -2-


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

     We 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. Repligen owns 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.

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

                                  RISK FACTORS

     YOU SHOULD CAREFULLY CONSIDER THE RISKS DESCRIBED BELOW BEFORE MAKING AN
INVESTMENT DECISION. ADDITIONAL RISKS AND UNCERTAINTIES THAT WE ARE UNAWARE OF
OR THAT WE CURRENTLY DEEM IMMATERIAL ALSO MAY BECOME IMPORTANT FACTORS THAT
AFFECT REPLIGEN.

     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.

IF WE DO NOT OBTAIN ADDITIONAL CAPITAL, 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 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 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


                                      -3-


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.

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.

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

     We must obtain 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:

     o obtain patent protection for our products and manufacturing processes;

     o preserve our trade secrets; and

     o operate without infringing the proprietary rights of third parties.

     There can be no assurance that any patent applications relating to our
products will be filed 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:

     o scope of the patent claims;

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

     o 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


                                      -4-



countries may be subject to opposition proceedings brought by third parties or
result in suits by Repligen which may be costly and result in adverse
consequences for Repligen.

     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 NOT BE YEAR 2000 COMPLIANT, AND AS A RESULT, MAY FACE, AND BE LIABLE FOR,
DATA CORRUPTION, COMPUTER FAILURE AND DISRUPTION OF OPERATIONS.

     Many existing computer systems and software products do not properly
recognize dates after December 31, 1999. This "Year 2000" problem could result
in miscalculations, data corruption, system failures or disruptions of
operations. We are subject to potential Year 2000 problems affecting our
computers' systems, our internal systems and the systems of our vendors and
scientific collaborators, any of which could have a material adverse affect on
our business operating results and financial conditions.

     Based on our assessments to date, we believe that our internal systems are
substantially Year 2000 compliant although there can be no assurance that Year
2000 errors or defects will not be discovered in our internal software systems
and, if such errors or defects are discovered, there can be no assurance that
the costs of making such systems Year 2000 compliant will not be material.

     Year 2000 errors or defects in the internal systems maintained by our
vendors of clinical trial collaborators could require us to incur significant
delays in our product development programs and unanticipated expenses to remedy
any problems or replace affected vendors. To date, no Y2K problems have been
detected by us and we are not aware of any problems with our suppliers or
collaborators.


                                      -5-


                                LEGAL PROCEEDINGS

     On July 17, 1998, Repligen filed a complaint against Bristol-Meyers 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. Repligen 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 Repligen's inventorship claim. Repligen continues 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. Repligen's failure to obtain shared ownership rights in the
patents may restrict Repligen's ability to commercialize CTLA4-Ig.

                                 USE OF PROCEEDS

     If all of the warrants are exercised, the net proceeds to Repligen from the
exercise of the warrants (after deducting estimated expenses in connection with
this offering) would range from $8,390,489 to $18,169,200, depending on when the
warrants are exercised. Repligen intends to use the net proceeds, if any, from
the sale of the common stock offered hereby for research and development,
including ongoing development of Repligen's technologies, pre-clinical and
clinical testing, and other costs associated with Repligen's product development
programs, for capital expenditures, and for working capital and other general
purposes. Repligen is offering the shares of common stock issuable upon exercise
of the warrants and filing this registration statement related thereto in order
to fulfill its contractual obligations to the warrantholders to maintain an
effective registration statement during the time that the warrants are
outstanding and exercisable.

                                    DILUTION

     The net tangible book value of Repligen, as of September 30, 1999, was
$12.4 million, or $.57 per share of common stock. Net tangible book value per
share represents the amount of total tangible assets less total liabilities
divided by the number of shares of common stock outstanding at that date.

     After giving effect to the sale by Repligen of the 2,007,050 shares of
common stock being offered hereby (assuming the exercise of all of the warrants)
at an assumed exercise price of $2.50 and $3.50 per share for the Exchange
Warrants and $9.00 for the Modified Original Warrants, our pro forma net
tangible book value as of September 30, 1999, would have been $20.8 million or
$.87 per share. This represents an immediate increase in pro forma net tangible
book value of $.30 per share to existing stockholders and an immediate dilution
of $1.63 to $8.13 per share to new investors. The following table illustrates
this per share dilution:


                                      -6-



                                                                                         
Assumed exercise price per share:
   Exchange Warrants exercisable for 570,086 shares........................                    $  2.50
   Exchange Warrants exercisable for 1,083,164 shares......................                       3.50
   Modified Original Warrants exercisable for 353,800 shares...............                       9.00
   Net tangible book value per share at September 30, 1999.................        $  .57
   Increase per share attributable to new investors........................           .30
                                                                                   ------
Pro forma net tangible book value per share after this offering............                        .87
                                                                                               -------
Dilution per share to new investors

   Holders of Exchange Warrants exercisable for $2.50 per share............                    $  1.63
   Holders of Exchange Warrants exercisable for $3.50 per share............                    $  2.63
   Holders of Modified Original Warrants exercisable for $9.00 per share...                    $  8.13


     Alternatively, if the warrants are adjusted such that the exercise price
becomes $8.00 per share for the Exchange Warrants and $14.00 per share for
the Modified Original Warrants, and after giving effect to the sale by
Repligen of the shares of common stock being offered hereby (assuming the
exercise of all the warrants) at such exercise prices, our pro forma net
tangible book value as of September 30, 1999, would have been $30.6 million
or $1.28 per share. This represents an immediate increase in pro forma net
tangible book value of $.71 per share to existing stockholders and an
immediate dilution of $6.72 to $12.72 per share to new investors. The
following table illustrates this per share dilution:


                                                                                           
Assumed exercise price per share:
   Exchange Warrants exercisable for 1,653,250 shares...........................                 $  8.00
   Modified Original Warrants exercisable for 353,800 shares....................                   14.00
   Net tangible book value per share at September 30, 1999......................        $  .57
   Increase per share attributable to new investors.............................           .71
                                                                                        ------
Pro forma net tangible book value per share after this offering.................                    1.28
                                                                                                 -------
Dilution per share to new investors
   Holders of Exchange Warrants exercisable for $8.00 per share.................                 $  6.72
   Holders of Modified Original Warrants exercisable for $14.00 per share.......                 $ 12.72


                DESCRIPTION OF THE WARRANTS/PLAN OF DISTRIBUTION

     In connection with the initial capitalization of Repligen Clinical
Partners, L.P. in February, 1992, Repligen issued to certain purchasers and the
sales agent related thereto and the agent's affiliates, units in Repligen
Clinical Partners. Each unit consisted of a Class A limited partnership interest
in Repligen Clinical Partners and a warrant (collectively, the "Original
Warrants") to purchase 2,900 shares of common stock of Repligen.

     In March 1994, Repligen offered to the holders of Original Warrants the
opportunity to exchange their Original Warrants for newly issued warrants (the
"Exchange Warrants"). Each holder of an Original Warrant was free to accept or
reject the exchange offer. Many


                                      -7-


warrantholders holding warrants to purchase shares of Repligen common stock
accepted the exchange offer. Some warrantholders were, however, ineligible to
participate in this exchange offer due to their failure to make payments on
promissory notes (the "Promissory Notes") which were originally issued by
certain of the warrantholders in favor of Repligen Clinical Partners to pay for
the units. Some warrantholders who were eligible to participate in the exchange
offer rejected the exchange offer and thus continued to hold Original Warrants.

     In connection with the March 1994 exchange of Original Warrants for
Exchange Warrants, the exercise price under the Exchange Warrants was reduced to
$9.00 (from $22.73) per share, subject to increase to $14.00 per share on the
date 90 days after Repligen notified the warrantholders holding the Exchange
Warrants that the closing price of Repligen common stock equaled or exceeded
$18.00 per share for 20 out of 30 consecutive trading days. The exercise period
under the Exchange Warrants was also extended to March 31, 2000. The holders of
Exchange Warrants also accepted a reduction in certain royalty rates that each
warrantholder would have received as a limited partner of Repligen Clinical
Partners, such royalties arising out of any sales of the drug (rPF4) being
developed by Repligen Clinical Partners. Acceptance of the Exchange Warrants
also resulted in pro rata reductions in certain other royalties which were
payable to such warrantholders as limited partners of Repligen Clinical
Partners.

     In March 1995, Repligen subsequently offered to the warrantholders an
opportunity to modify the Original Warrants and the Exchange Warrants that
remained outstanding. With respect to each holder of an outstanding Original
Warrant or Exchange Warrant who was required to make and did make the fourth
installment payment to Repligen Clinical Partners pursuant to the Promissory
Notes, such holders (along with the other warrantholders who did not have
outstanding Promissory Notes) were free to accept or reject the proposed
modifications.

     With respect to the Original Warrants, the modifications provided that the
terms of the Original Warrants would be modified (the "Modified Original
Warrants") so as to be identical to those of the Exchange Warrants issued in the
1994 exchange offer (i.e., an extension of the exercise period by one year to
March 31, 2000 and a per share exercise price reduction from $22.73 to $9.00,
subject to escalation to $14.00 per share 90 days after Repligen notified
holders thereof that the closing price of Repligen common stock equaled or
exceeded $18.00 per share for 20 out of 30 consecutive trading days).

     With respect to the Exchange Warrants being modified in March 1995, the
exercise price of the first 1,000 shares of Repligen common stock issuable upon
exercise thereof was reduced from $9.00 per share to $2.50 per share, and the
exercise price for the remaining 1,900 shares of Repligen common stock issuable
upon exercise thereof was reduced from $9.00 per share to $3.50 per share. The
exercise price of the modified Exchange Warrants was subject to increase to
$8.00 per share on the date 90 days after Repligen notified holders thereof that
the closing price of Repligen common stock equaled or exceeded $12.00 per share
for 20 out of 30 consecutive trading days. Lastly, Repligen further modified the
Exchange Warrants by extending the expiration period for the Exchange Warrants
by one year, to March 31, 2001. The proposed modifications did not include
further reductions in any royalty rates payable by Repligen. The difference in
treatment between the Original Warrants and Exchange Warrants in the March 1995
modification was due to the fact that holders of Original Warrants were those
warrantholders


                                      -8-


who did not accept the 1994 exchange offer and consequently had a right to
receive higher royalty rates than holders of Exchange Warrants.

     The exercise price and the number of shares issuable upon exercise of each
warrant will be appropriately adjusted in the event of stock splits, stock
combinations, stock dividends, reclassifications or rights offerings involving
common stock or distributions of certain subscription rights, warrants or
evidences of indebtedness to holders of common stock. Repligen will not issue
fractional shares upon exercise of the warrants. Instead Repligen will pay to
the holder a cash adjustment equal to the amount of fractional shares based on
the closing price of common stock as reported on the Nasdaq National Market on
the date of exercise.

     With certain exceptions, in case of any reclassification or capital
reorganization, or in case of any consolidation or merger of Repligen or any
sale, lease, transfer or conveyance of all or substantially all of the assets of
Repligen, each holder of a warrant hereunder shall have the right, upon
subsequent exercise of such warrant, to purchase the kind and amount of shares
of stock or other securities and property receivable upon such reclassification,
capital reorganization, consolidation, merger, sale, lease, transfer or
conveyance by a holder of the number of shares of common stock that might have
been received upon the exercise of such warrant immediately prior to such event,
and the exercise price of such warrants will be appropriately adjusted. The
warrants do not confer upon the warrantholders any rights as stockholders of
Repligen.

     As of December 1, 1999, Modified Original Warrants and Exchange Warrants to
purchase 353,800 shares of Repligen common stock were issued and outstanding and
such warrants will expire on March 31, 2000. As of December 1, 1999, modified
Exchange Warrants to purchase 1,653,250 shares of Repligen common stock were
issued and outstanding and such warrants will expire on March 31, 2001.

     Repligen had previously filed a registration statement in April 1994 to
register the issuance of common stock upon exercise of the warrants issued as
part of the units issued by Repligen Clinical Partners. Because that
registration statement became inactive (due to in part because the exercise
price of the warrants was significantly higher than the fair market value of the
underlying common stock), Repligen is filing this registration statement to
register the issuance of shares of common stock upon exercise of the Modified
Original Warrants and the modified Exchange Warrants in compliance with its
contractual obligations to the warrantholders to maintain an effective
registration statement during the period the warrants are outstanding and
exercisable.

WARRANT EXERCISE PROCEDURE

     Repligen has not engaged any underwriter, broker or dealer in connection
with the issuance of the shares to the warrantholders and will pay no
commissions thereon. In order to effectively exercise the warrants and purchase
the shares issuable upon exercise thereof, a warrantholder must comply with the
procedure described below. Failure to follow this procedure may result in either
a delay in the warrantholder's receipt of a stock certificate or an ineffective
exercise and return to the warrantholder of the materials submitted for
exercise.

     The warrantholder must use the "Purchase Form" attached to the end of the
certificate representing the warrant ("the Warrant Certificate"). The
warrantholder must specify the number of


                                      -9-


shares being purchased and the exercise payment being remitted. The
warrantholder must sign the Purchase Form where indicated and the signature must
conform with the manner in which the Warrant Certificate is held as appears on
the warrant register maintained by Repligen (the "Warrant Register").

     Each holder of a Warrant Certificate must deliver the completed Purchase
Form, and cash or a check drawn to the order of "Repligen Corporation" for the
exact amount of the exercise price for the shares being purchased to the
Director of Finance, Repligen Corporation, 117 Fourth Avenue Needham, MA 02494.
In addition, to satisfy Internal Revenue Service requirements, the exercising
warrantholder should provide Repligen with his or her social security number or
taxpayer identification number. Also, an exercising warrantholder should provide
Repligen with the address to which he, she or it wants the share certificate
delivered. Each warrantholder should direct any questions concerning exercise of
the warrants to the Director of Finance at the address listed above.

     Upon receipt of the Warrant Certificate, including the completed Purchase
Form, appropriate payment for the shares being purchased, and the exercising
warrantholder's social security or taxpayer identification number, Repligen will
instruct its transfer agent to issue a share certificate in the warrantholder's
name (or other name if so designated) for the shares being purchased. The
transfer agent will send the share certificate to the exercising warrantholder
at the address indicated at the time of exercise; otherwise the transfer agent
will send the share certificate to the address listed on the Warrant Register.
Unless otherwise agreed to by Repligen, Repligen will send all share
certificates by first class mail with an estimated delivery time of about four
weeks from the date of Repligen's receipt of the exercise material.

     Upon receipt of the Warrant Certificate, including the completed Purchase
Form, appropriate payment for the shares being purchased, and the exercising
warrantholder's social security or taxpayer identification number, the
warrantholder will become the holder of record of those shares being purchased
and shall have all of the rights of a stockholder, including the right to vote
the shares.

PARTIAL EXERCISE

     A warrantholder may exercise his or her Warrant Certificate for only part
of the shares the Warrant Certificate represents. If a warrantholder exercises
only a part of the shares represented by a Warrant Certificate, then in addition
to the share certificate for the exercised shares, he or she will receive a new
Warrant Certificate, dated the date of his or her original Warrant Certificate,
representing those shares still subject to exercise.

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


                                      -10-


                                  LEGAL MATTERS

     The validity of the shares of common stock of Repligen offered hereby will
be passed upon for Repligen 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 September 30, 1999
     and June 30, 1999;

4.   Current reports on Form 8-K filed October 6, 1999, March 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

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.


                                      -11-


     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.


                                      -12-


                                     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....$   4,799.30
Blue Sky Fees and Expenses................................$   1,000.00
Accounting Fees and Expenses..............................$   3,000.00
Legal Fees and Expenses...................................$  10,000.00
Transfer Agent fees and expenses..........................$   5,000.00
Miscellaneous.............................................$   6,200.70
                                                          ------------
                  TOTAL...................................$  30,000.00
                                                          ============


     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.


                                      II-1


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               Form of Modified Limited Partner Warrant, dated as of
                  February 28, 1992 (referred to as the Modified Original
                  Warrant) (filed as Exhibit 4.3 to Repligen Corporation's
                  Annual Report on Form 10-K for the year ended March 31, 1998
                  and incorporated herein by reference).

+4.2              Form of Amended, Modified and Restated Warrant (referred to as
                  the Exchange Warrant).

+5            --  Legal Opinion of Testa, Hurwitz & Thibeault, LLP

+23.1         --  Consent of Arthur Andersen LLP

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

+24           --  Power of Attorney


- ------------------
+ filed herewith

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


     (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


                                   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 January
28, 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.



NAME                                      CAPACITY                                    DATE

                                                                                
/s/ Walter C. Herlihy                     President and Chief Executive               January 28, 2000
- --------------------------------------    Officer, Chief Financial Officer
Walter C. Herlihy                         and Director (principal executive,
                                          financial and accounting officer)

/s/ Alexander Rich, M.D.                  Co-Chairman of the Board of                 January 28, 2000
- --------------------------------------    Directors
Alexander Rich, M.D.


                                      II-4


/s/ Paul Schimmel, Ph.D.                  Co-Chairman of the Board of                 January 28, 2000
- --------------------------------------    Directors
Paul Schimmel, Ph.D.

/s/ Robert J. Hennessey                   Director                                    January 28, 2000
- --------------------------------------
Robert J. Hennessey

/s/ G. William Miller                     Director                                    January 28, 2000
- --------------------------------------
G. William Miller



                                      II-5


                                INDEX TO EXHIBITS



EXHIBIT NO.       ITEM AND REFERENCE

           
4.1               Form of Modified Limited Partner Warrant, dated as of February
                  28, 1992 (referred to as the Modified Original Warrant) (filed
                  as Exhibit 4.3 to Repligen Corporation's Annual Report on Form
                  10-K for the year ended March 31, 1998 and incorporated herein
                  by reference).

+4.2              Form of Amended, Modified and Restated Warrant (referred to as
                  the Exchange Warrant).

+5            --  Legal Opinion of Testa, Hurwitz & Thibeault, LLP

+23.1         --  Consent of Arthur Andersen LLP

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

+24           --  Power of Attorney


- -----------------
+ filed herewith


                                      II-6