SECURITIES AND EXCHANGE COMMISSION

                             Washington, D.C. 20549

                                    FORM 10-Q

             |X| QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF
                       THE SECURITIES EXCHANGE ACT OF 1934

                  For the quarterly period ended June 30, 1999

                                       OR

            |_| TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF
                       THE SECURITIES EXCHANGE ACT OF 1934

   For the transition period from ____________________ to ____________________

                         Commission File Number 0-14656

                              REPLIGEN CORPORATION
             (exact name of registrant as specified in its charter)

                Delaware                                     04-2729386
    (State or other jurisdiction of                       (I.R.S. Employer
     incorporation or organization)                      Identification No.)

           117 Fourth Avenue
         Needham, Massachusetts                                 02494
(Address of principal executive offices)                      (Zip Code)

       Registrant's telephone number, including area code: (781) 449-9560

        -----------------------------------------------------------------
         (Former name, former address and former fiscal year, if changed
                               since last report.)

Indicate by check mark whether the registrant (1) has filed all reports required
to be filed by Section 13 or 15 (d) of the Securities Exchange Act of 1934
during the preceding 12 months (or for such shorter period that the registrant
was required to file such reports), and (2) has been subject to such filing
requirements for the past 90 days. Yes |X| No |_|

Indicate the number of shares outstanding of each of the issuer's classes of
common stock, as of July 31, 1999.

     Common Stock, par value $.01 per share                 21,868,085
     --------------------------------------              ----------------
               Class                                     Number of Shares



                                        2


                              REPLIGEN CORPORATION

                                      INDEX



                                                                                      PAGE
                                                                                      ----
                          PART I. FINANCIAL INFORMATION

                                                                                   
Item 1.  Financial Statements

      Condensed Consolidated Balance Sheets as of June 30, 1999 (Unaudited)
      and March 31, 1999 (Audited)                                                       4

      Condensed Consolidated Statements of Operations (Unaudited) for
      the Three Months Ended June 30, 1999 and 1998                                      5

      Condensed Consolidated Statement of Cash Flows (Unaudited)
      for the Three Months Ended June 30, 1999 and 1998                                  6

      Notes to Condensed Consolidated Financial Statements (Unaudited)                   7

Item 2.  Management's Discussion and Analysis of Financial Condition
         and Results of Operations                                                      10

                           PART II. OTHER INFORMATION

Item 1.  Legal Proceedings                                                              14

Item 2.  Changes in Securities                                                          14

Item 3.  Defaults Upon Senior Securities                                              None

Item 4.  Submissions of Matters to a Vote of Security Holders                         None

Item 5.  Other Information                                                            None

Item 6.  Exhibits and Reports on Form 8-K                                               15

Signature                                                                               17

Exhibit Index                                                                           18

Exhibits                                                                                19



                                       3


PART I. FINANCIAL INFORMATION

ITEM I. FINANCIAL STATEMENTS

                              REPLIGEN CORPORATION
                      CONDENSED CONSOLIDATED BALANCE SHEETS



                                                         June 30, 1999      March 31, 1999
                                                         -------------      -------------
     ASSETS                                               (Unaudited)          (Audited)
                                                                      
Current assets:
  Cash and cash equivalents                              $  11,817,472      $   3,250,751
  Accounts receivable                                          404,370            429,720
  Inventories                                                  581,762            630,329
  Prepaid expenses and other current assets                    267,704            181,617
                                                         -------------      -------------
    Total current assets                                    13,071,308          4,492,417

Property and equipment, at cost:
  Equipment                                                    969,471            944,644
  Furniture and fixtures                                       139,385            101,376
  Leasehold improvements                                       467,368            460,319
                                                         -------------      -------------
                                                             1,576,224          1,506,339
  Less: accumulated depreciation and amortization              936,053            862,934
                                                         -------------      -------------
                                                               640,171            643,405

Other assets, net                                               81,411             88,472
                                                         -------------      -------------
                                                         $  13,792,890      $   5,224,294
                                                         =============      =============

            LIABILITIES AND STOCKHOLDERS' EQUITY

Current liabilities:
  Accounts payable                                       $     352,800      $     268,708
  Accrued expenses                                             338,244            313,926
  Unearned income                                               27,384             49,969
                                                         -------------      -------------
     Total current liabilities                                 718,428            632,603

Commitments and contingencies                                       --                 --

Stockholders' equity:
  Preferred stock, $.01 par value --
    authorized -- 5,000,000 shares --
    outstanding - none                                              --                 --
  Common stock, $.01 par value --
   authorized -- 30,000,000 shares --
   outstanding - 21,868,085 shares
   at June 30, 1999 and 18,264,285
   shares at March 31, 1999                                    218,680            182,642
  Additional paid-in capital                               140,141,927        131,272,607
  Accumulated deficit                                     (127,286,145)      (126,863,558)
                                                         -------------      -------------
     Total stockholders' equity                             13,074,462          4,591,691
                                                         -------------      -------------

                                                         $  13,792,890      $   5,224,294
                                                         =============      =============


     See accompanying notes to condensed consolidated financial statements.


                                       4


                              REPLIGEN CORPORATION
                 CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS
                                   (Unaudited)



                                                                      Three Months Ended
                                                                      ------------------
                                                                 June 30,1999     June 30, 1998
                                                                 ------------      ------------
                                                                             
Revenues:
  Research and development                                       $    378,502      $    268,406
  Product                                                             232,470           229,138
  Investment income                                                    46,538            61,691
  Other                                                                30,658            33,188
                                                                 ------------      ------------
                                                                      688,168           592,423
                                                                 ------------      ------------

Costs and expenses:
  Research and development                                            488,203           466,069
  Selling, general and administrative                                 426,169           356,932
  Cost of products sold                                               196,383           112,282
                                                                 ------------      ------------
                                                                    1,110,755           935,283
                                                                 ------------      ------------

Net loss                                                         $   (422,587)     $   (342,860)
                                                                 ============      ============

Basic and diluted net loss per share                             $      (0.02)     $      (0.02)
                                                                 ============      ============

Basic and diluted weighted average common shares outstanding       18,744,863        18,001,785
                                                                 ============      ============


     See accompanying notes to condensed consolidated financial statements.


                                       5


                              REPLIGEN CORPORATION
                 CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS
                                   (Unaudited)



                                                                         Three Months Ended June 30,
                                                                        ------------------------------
                                                                            1999              1998
                                                                        ------------      ------------
                                                                                    
Cash flows from operating activities:
   Net loss                                                             $   (422,587)     $   (342,860)
   Adjustments to reconcile net loss to net cash
   used in operating activities -
     Depreciation and amortization                                            73,119            64,213

Changes in assets and liabilities -
   Accounts receivable                                                        25,349          (193,724)
   Inventories                                                                48,567           (37,111)
   Prepaid expenses and other current assets                                 (86,087)           44,512
   Accounts payable                                                           84,094            (1,208)
   Accrued expenses                                                           24,318           116,491
   Unearned income                                                           (22,585)          (33,332)
                                                                        ------------      ------------

     Net cash used in operating activities                                  (275,812)         (383,019)
                                                                        ------------      ------------

Cash flows from investing activities:
   Purchases of property and equipment, at cost                              (69,886)          (55,953)
   Changes in other assets                                                     7,061                --
                                                                        ------------      ------------

     Net cash used in investing activities                                   (62,825)          (55,953)
                                                                        ------------      ------------

Cash flows from financing activities:
   Net proceeds from the issuance of common stock and warrants,
   net of issuance costs                                                   8,905,358                --
                                                                        ------------      ------------
     Net cash provided by financing activities                             8,905,358                --
                                                                        ------------      ------------

Net increase (decrease) in cash and cash equivalents                       8,566,721          (438,971)
Cash and cash equivalents, beginning of period                             3,250,751         4,725,544
                                                                        ------------      ------------

Cash and cash equivalents, end of period                                $ 11,817,472      $  4,286,573
                                                                        ============      ============


     See accompanying notes to condensed consolidated financial statements.


                                       6


                              REPLIGEN CORPORATION
              NOTES TO CONDENSED CONSOLIDATED FINANCIAL STATEMENTS
                                   (Unaudited)

      1.    Basis of Presentation

      The condensed consolidated financial statements included herein have been
prepared by Repligen Corporation (the "Company" or "Repligen"), pursuant to the
rules and regulations of the Securities and Exchange Commission for quarterly
reports on Form 10-Q and do not include all of the information and footnote
disclosures required by generally accepted accounting principles. These
financial statements should be read in conjunction with the audited financial
statements and notes thereto included in the Company's Form 10-K for the year
ended March 31, 1999.

      In the opinion of management, the accompanying unaudited financial
statements include all adjustments, consisting of only normal, recurring
adjustments, necessary to present fairly, the consolidated financial position,
results of operations and cash flows. The results of operations for the interim
periods presented are not necessarily indicative of results to be expected for
the entire year.

      The preparation of financial statements in conformity with generally
accepted accounting principles requires management to make estimates and
assumptions that affect the reported amounts of assets and liabilities and
disclosure of contingent assets and liabilities at the date of the financial
statements and the reported amounts of revenues and expenses during the
reporting period. Actual results could differ from those estimates.

      2.    Net Loss Per Share

      The Company has adopted Statement of Financial Accounting Standards (SFAS)
No. 128, Earnings per Share, effective December 15, 1998. SFAS No. 128
establishes standards for computing and presenting earnings per share and
applies to entities with publicly held common stock or potential common stock.
The Company has applied the provisions of SFAS No. 128, retroactively to all
periods presented. Basic and diluted net loss per share represents net loss
divided by the weighted average number of common shares outstanding during the
period. The dilutive effect of the potential common shares consisting of
outstanding stock options and warrants is determined using the treasury stock
method in accordance with SFAS No. 128. Diluted weighted average shares
outstanding at June 30, 1999 and 1998 excluded the potential common shares from
warrants and stock options because to do so would be antidilutive for the
periods presented. At June 30, 1999, there are 1,335,491 options outstanding
with a weighted average exercise price of $1.82 and 3,207,050 warrants
outstanding with a weighted average exercise price of $3.19.

      3.    Cash and Cash Equivalents


                                       7


      The Company considers highly liquid investments purchased with original
maturities at the date of acquisition of three months or less to be cash
equivalents. Cash equivalents consist of the following at June 30, 1999 and
March 31, 1999:

                                                     Three Months Ended
                                               June 30,1999      March 31, 1999
                                               ------------      --------------
U.S. Government and Agency securities ......    $ 5,585,426       $ 1,197,624
Commercial paper ...........................      5,185,168         1,136,119
Money markets ..............................        969,188           802,755
Cash .......................................         77,690           114,253
                                                -----------       -----------
   Total cash and cash equivalents .........    $11,817,472       $ 3,250,751
                                                ===========       ===========

      4.    Inventories

      Inventories are stated at the lower of cost (first-in, first-out) or
market and consist of the following:

                                            June 30, 1999     March 31,1999
                                             (Unaudited)
      Raw materials and work-in-process        $420,726         $412,480
      Finished goods                            161,036          217,849
                                               --------         --------
           Total                               $581,762         $630,329
                                               ========         ========

      Work in process and finished goods inventories consist of material, labor,
outside processing costs and manufacturing overhead.

      5.    Comprehensive Income

      Effective January 1, 1998, the Company adopted SFAS No. 130 Reporting
Comprehensive Income, effective January 1, 1998. SFAS No. 130 establishes
standards for reporting and display of comprehensive income and its components
in financial statements. Comprehensive income includes all changes in equity
during a period except those resulting from investments by owners and
distributions to owners. The comprehensive net loss is the same as net loss for
all periods presented.

      6.    Disclosures about Segments of an Enterprise and Significant
            Customers

      The Company has adopted SFAS No. 131, Disclosures about Segments of an
Enterprise and Related Information, in the fiscal year ended March 31, 1999.
SFAS No. 131 establishes


                                       8


standards for reporting information regarding operating segments in annual
financial statements and requires selected information for those segments to be
presented in interim financial reports issued to stockholders. SFAS No. 131 also
establishes standards for related disclosures about products and services and
geographic areas. Operating segments are identified as components of an
enterprise about which separate discrete financial information is available for
evaluation by the chief operating decision maker, or decision making group, in
making decisions now to allocate resources and assess performance. To date, the
Company has viewed its operations and manages its business as principally one
operating segment. As a result, the financial information disclosed herein,
represents all of the material financial information related to the Company's
principal operating segment.

      The following table represents the Company's revenue by country:

                                            Three Months Ended June 30,
                                    -----------------------------------------
                                      1999             1998           1997
                                      ----             ----           ----
         United States                 56%              57%            85%
         Sweden                        25%               4%             1%
         United Kingdom                14%              12%             8%
         Other                          5%              27%             6%
         Total                        100%             100%           100%

      During the three months ended June 30,1999, there were two significant
customers which accounted for approximately 25% and 14% of the Company's
revenues or $172,000 and $96,000. The related accounts receivable for these two
customers at June 30, 1999 was fully paid and $96,000, respectively.

      7.    Sale of Securities

      Pursuant to two stock purchase agreements dated as of April 30, 1999 and
May 14, 1999, respectively, by and among Repligen and the respective parties
thereto, Repligen issued to the parties thereto an aggregate of 3,600,000 shares
of Common Stock of Repligen for an aggregate purchase price of $9 million. Based
on the representations of the investing parties and a reasonable belief of
Repligen that all such parties were "accredited" (as such term is defined in
Rule 501 of the Securities Act of 1933) and that the parties were acquiring the
shares of Common Stock of Repligen for investment and not with a view to the
distribution thereof, Repligen consummated a private placement of the 3,600,000
shares of Common Stock of Repligen pursuant to Regulation D, Rule 506 of the
Securities Act of 1933. Repligen closed the private placement transaction on
June 23, 1999. There were no underwriters involved in such private placement
transaction. Repligen will use the proceeds from the private placement
transaction for working capital and other general corporate purposes. Repligen
filed a registration statement with the Securities and Exchange Commission on
Form S-3 on June 16, 1999 for the resale of the 3,600,000 shares of Common Stock
of Repligen sold to the parties in the private placement transaction. The


                                       9


Securities and Exchange Commission declared such resale registration statement
effective on June 23, 1999.

                 ITEM 2. MANAGEMENT'S DISCUSSION AND ANALYSIS OF
                  FINANCIAL CONDITION AND RESULTS OF OPERATIONS

      Cautionary Statement Regarding Forward-Looking Statements

      Statements in this Quarterly Report on Form 10-Q as well as oral
statements that may be made by the Company or by officers, directors or
employees of the Company acting on the Company's behalf, that are not
historical facts constitute "forward-looking statements" within the meaning
of Section 21E of the Securities Exchange Act of 1934. Such forward-looking
statements involve known and unknown risks, uncertainties and other factors
that could cause the actual results of the Company to be materially different
from the historical results or from any results expressed or implied by such
forward-looking statements. The Company's future operating results are
subject to risks and uncertainties and are dependent upon many factors,
including, without limitation, the Company's ability to (i) meet its working
capital and future liquidity needs, (ii) successfully implement its strategic
growth strategies, (iii) understand, anticipate and respond to rapidly
changing technologies and market trends, (iv) develop, manufacture and
deliver high quality, technologically advanced products on a timely basis to
withstand competition from competitors which may have greater financial,
information gathering and marketing resources than the Company, (v) obtain
and protect licensing and intellectual property rights necessary for the
Company's technology and product development on terms favorable to the
Company, (vi) recruit and retain highly talented professionals in a
competitive job market, (vii) realize future revenues, (viii) maintain a
timeline for clinical activity, (ix) obtain successful results of pending or
future clinical trials, (x) continue to establish collaborative arrangements
with third parties, and (xi) compete against the biotechnology and
pharmaceutical industries. Further information on potential factors that
could affect the Company's financial results are included in filings made by
the Company from time to time with the Securities and Exchange Commission
included in the section entitled "Risk Factors" contained in the Company's
Annual Report on Form 10-K for the fiscal year ended March 31, 1999 (File
No.000-14656).

      Overview

      Repligen develops new drugs for the treatment of neurological disease,
organ transplantation and cancer. To expand our drug development program, on
March 9, 1999, Repligen 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 communication and social skills, negative
behavior, irregular sleep patterns and diminished ability to learn. 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 United States Food and Drug Administration for use in
diagnosing problems with pancreatic function. Recent anecdotal reports indicate
that secretin may have beneficial effects in treating


                                       10


autism, including improvements in sleep, digestive function, speech 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. Currently there
are no drugs approved by the FDA for the treatment of autism.

      Repligen also is 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
December 1998, investigators from the Dana-Farber Cancer Institute, a research
hospital in Boston, reported that treatment of bone marrow from a family member
who was taking CTLA4-Ig substantially reduced Graph Versus Host Disease in
twelve transplant patients. Repligen intends to further evaluate CTLA4-Ig in
bone marrow transplants for leukemia.

      Repligen develops, manufactures and markets 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. In December 1998, Repligen entered
into a ten year relationship to supply recombinant Protein A to Amersham
Pharmacia Biotech, a leading supplier to the biopharmaceutical market.

      Results of Operations

      Revenues

      Total revenues for the three month period ended June 30, 1999 and 1998
were approximately $688,000 and $592,000, respectively, an increase of
approximately $96,000 or 16%. This increase was largely attributable to
increased research and development revenue.

      Research and development revenues for the three month period ended June
30, 1999 and 1998 were approximately $379,000 and $268,000, respectively, an
increase of approximately $111,000 or 41%. This increase was largely
attributable to a licensing payment received during the three months ended June
30, 1999 offset by decreased revenues associated with research collaborations on
its drug discovery programs during the


                                       11


three months ended June 30, 1999.

      Product revenues for the three month period ended June 30, 1999 and 1998
were approximately $232,000 and $229,000, respectively, an increase of $3,000 or
1%. This increase is attributable to increased sales of Protein A products
offset by decreased sales of reagent products.

      Investment income for the three month period ended June 30, 1999 and
1998 was approximately $47,000 and $62,000, respectively, a decrease of
approximately $15,000 or 24%. This decrease is largely attributable to higher
average funds available for investment and higher interest rates during the
three months ended June 30, 1998. The Company completed its common stock
financing on June 23, 1999.

      Other revenues for the three month period ended June 30, 1999 were
approximately $31,000, a decrease of $2,000 or 6% from the comparable period
ended June 30, 1998. This decrease is primarily due to the sale of equipment
held by Repligen reported as other income in fiscal 1999.

      Expenses

      Total expenses for the three month period ended June 30, 1999 and 1998
increased to approximately $1,111,000 from $935,000, an increase of $176,000 or
18%.

      Research and development expenses for the three month period ended June
30, 1999 and 1998 were approximately $488,000 and $466,000, respectively, an
increase of $22,000 or 5%. This increase reflects increased staffing in research
and development as Repligen expands its investment in its drug development
programs.

      Selling, general and administrative expenses for the three months ended
June 30, 1999 and 1998 were approximately $426,000 and $357,000, respectively,
an increase of $69,000 or 19%. This increase is attributable to increased costs
in administrative salaries and associated benefits.

      Cost of products sold for the three months ended June 30, 1999 and 1998
were approximately $196,000 and $112,000, respectively, an increase of $84,000,
or 75%. Cost of products sold in the three months ended June 30, 1999 and 1998
were 85% and 49%, respectively, of product revenues. This increase is largely
attributable to costs associated with the startup of its manufacturing contract
for AP Biotech during the three month period ended June 30, 1999.

      Liquidity and Capital Resources

      Repligen's total cash and cash equivalents increased to $11,817,000 at
June 30, 1999 from $3,251,000 at March 31, 1999. This increase of $8,566,000
reflects $8,900,000 of proceeds resulting from the sale of Common Stock to
certain investors through a private placement that closed during the three
months ended June 30, 1999, offset by a net


                                       12


loss from operations incurred during the three month period ended June 30, 1999
of approximately $423,000, an increase in prepaid expenses of $86,000 and
capital expenditures of $70,000, offset in part by the increase in accrued
expenses and accounts payable of $108,000 and accounts receivable of $49,000.
Working capital increased to $12,353,000 at June 30, 1999 from $3,860,000 at
March 31, 1999.

      Repligen has entered into agreements with a number of collaborative
partners and licensees. Under the terms of these agreements, Repligen may be
eligible to receive research support, additional milestones or royalty revenue
if these collaborations result in clinical evaluation and commercialization of
products developed. Repligen can not be certain of the continuation of these
collaborations or of receiving any future payments related to these agreements.

      While the cost of operations is anticipated to increase in fiscal 2000 as
Repligen expands its investment in proprietary product development, Repligen
believes that the private placement financing, yielding an aggregate of
$8,900,000 in net proceeds will provide sufficient funding to satisfy its
working capital and capital expenditure requirements for the next twenty-four
months. Should Repligen need to secure additional financing to meet its future
liquidity requirements, Repligen may not be able to secure such financing, or
obtain such financing on favorable terms because of the volatile nature of the
biotechnology market place.

      Year 2000

      Repligen has undertaken an initial review of its information technology
computer systems and it believes that the Year 2000 problem does not pose
significant operational problems to its information technology systems. The
majority of Repligen's software and computer equipment has been purchased within
the last five years from third-party vendors who have already provided upgrades
intended to bring their products into Year 2000 compliance. Repligen has begun
to address the small number of internal systems that are not yet Year 2000
compliant, and expects full compliance by the end of 1999. Repligen currently
believes that the costs of addressing these issues should not exceed $50,000 and
will not have a material adverse impact on Repligen's financial position.

      Repligen has recently begun interviewing various third parties, including
vendors and suppliers of Repligen, to determine their exposure to Year 2000
issues, their anticipated risks and responses to those risks. To date, the third
parties that have been contacted have indicated that their hardware or software
is or will be Year 2000 compliant in a time frame that meets Repligen's
requirements. Even with the vendor compliance however, Repligen intends to
continue to assess its exposure to Year 2000 noncompliance on the part of any of
its material vendors. There can be no assurance that the vendor's systems will
be Year 2000 compliant in a time frame satisfactory to Repligen.


                                       13


      Repligen does not have a contingency plan in the event Year 2000
compliance cannot be achieved in a timely manner. A contingency plan will be
developed immediately upon completion of Repligen's Year 2000 compliance
assessment.

      Item 1. LEGAL PROCEEDINGS

      On July 17, 1998, Repligen filed a complaint 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 Bristol-Myers 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 Bristol-Myers patents and we
intend to continue to pursue the correction of inventorship. There can be no
assurances that litigation will conclude in a result beneficial to Repligen, and
failure to obtain shared ownership rights in the patents may restrict Repligen's
ability to commercialize CTLA4-Ig.

      Item 2. CHANGES IN SECURITIES

      Pursuant to two stock purchase agreements dated as of April 30, 1999 and
May 14, 1999, respectively, by and among Repligen and the respective parties
thereto, Repligen issued to the parties thereto an aggregate of 3,600,000 shares
of Common Stock of Repligen for an aggregate purchase price of $9 million. Based
on the representations of the investing parties and a reasonable belief of
Repligen that all such parties were "accredited" (as such term is defined in
Rule 501 of the Securities Act of 1933) and that the parties were acquiring the
shares of Common Stock of Repligen for investment and not with a view to the
distribution thereof, Repligen consummated a private placement of the 3,600,000
shares of Common Stock of Repligen pursuant to Regulation D, Rule 506 of the
Securities Act of 1933. Repligen closed the private placement transaction on
June 23, 1999. There were no underwriters involved in such private placement
transaction. Repligen will use the proceeds from the private placement
transaction for working capital and other general corporate purposes. Repligen
filed a registration statement with the Securities and Exchange Commission on
Form S-3 on June 16, 1999 for the resale of the 3,600,000 shares of Common Stock
of Repligen sold to the parties to the private placement transaction. The
Securities and Exchange Commission declared such resale registration statement
effective on June 23, 1999.


                                       14


      Item 6. Exhibits and Reports on Form 8-K

      (a)   Exhibits


      EXHIBIT            DESCRIPTION

      3.1                Restated Certificate of Incorporation, dated June 30,
                         1992 and filed July 13, 1992 (filed as Exhibit 4.12
                         to Repligen Corporation's Annual Report on Form 10-K
                         File No. 0-14656 for the year ended March 31, 1993
                         and incorporated herein by reference).

      3.2                By-laws (filed as Exhibit 3.4 to Repligen
                         Corporation's Form S-1 Registration Statement No.
                         33-3959 and incorporated herein by reference).

      4.1                Stock Purchase Agreement dated as of April 30, 1999,
                         by and among Repligen Corporation and Wellington
                         Management Company, LLP, as Investment Advisor to the
                         investors listed on Schedule I thereto (previously
                         filed as Exhibit 4.1 to Repligen's current report on
                         Form 8-K on May 17, 1999 and incorporated herein by
                         reference).

      4.2                Stock Purchase Agreement dated as of May 14, 1999, by
                         and among Repligen Corporation and the investors
                         listed on the Schedule I thereto (previously filed as
                         Exhibit 4.2 to Repligen's current report on Form 8-K
                         on May 17, 1999 and incorporated herein by
                         reference).

      10.1*              Supply Agreement dated as of May 26, 1999 by and
                         between Repligen Corporation and Amersham Pharmacia
                         Biotech AB (with certain confidential information
                         deleted) (filed herewith).

      27.1               Financial Data Schedule (filed herewith)

      * The Appendixes to the Supply Agreement, included as Exhibit 10.1 hereto,
are not being filed herewith. The Supply Agreement filed herewith contains a
list briefly identifying the contents of all appendixes to such document. The
Company undertakes to furnish a copy of any omitted appendix to the Commission
upon request (except that Appendix C and D thereto shall


                                       15


remain confidential). Pursuant to Item 601 (b) (2) of Regulation S-K, set forth
below is a list of the omitted appendixes.

    SUPPLY AGREEMENT                     (EXHIBIT 10.1 HERETO)

    Appendix A                           Biotech IPA Products

    Appendix B                           Certificate of Analysis

    Appendix C                           Price Schedule for First Contract Year

    Appendix D                           New Product

      (b)   Reports on Form 8-K

      1.    Current Report on Form 8-K/A filed with the Securities and Exchange
            Commission on June 15, 1999 (amending Current Report on Form 8-K
            filed March 24, 1999) (description of transaction relating to
            acquisition of all rights to certain patent applications).

      2.    Current Report on Form 8-K filed with the Securities and Exchange
            Commission on May 17, 1999 (description of private placement
            transaction relating to sale of common stock of the Company for an
            aggregate purchase price of $9 million).


                                       16


                                    SIGNATURE

      Pursuant to the requirements of the Securities Exchange Act of 1934, the
registrant has duly caused this report to be signed on its behalf by the
undersigned thereunto duly authorized.


                                    REPLIGEN CORPORATION
                                    (Registrant)


Date: August 13, 1999           By: /s/ Walter C. Herlihy
                                    ------------------------------
                                    Chief Executive Officer and President,
                                    Principal Financial and Accounting Officer


                                       17


                              Repligen Corporation
                                  Exhibit Index

   EXHIBIT           DESCRIPTION

   3.1               Restated Certificate of Incorporation, dated June 30, 1992
                     and filed July 13, 1992 (filed as Exhibit 4.12 to Repligen
                     Corporation's Annual Report on Form 10-K File No. 0-14656
                     for the year ended March 31, 1993 and incorporated herein
                     by reference).

   3.2               By-laws (filed as Exhibit 3.4 to Repligen Corporation's
                     Form S-1 Registration Statement No. 33-3959 and
                     incorporated herein by reference).

   4.1               Stock Purchase Agreement dated as of April 30, 1999, by and
                     among Repligen Corporation and Wellington Management
                     Company, LLP, as Investment Advisor to the investors listed
                     on Schedule I thereto (previously filed as Exhibit 4.1 to
                     Repligen's current report on Form 8-K on May 17, 1999 and
                     incorporated herein by reference).

   4.2               Stock Purchase Agreement dated as of May 14, 1999, by and
                     among Repligen Corporation and the investors listed on the
                     Schedule I thereto (previously filed as Exhibit 4.2 to
                     Repligen's current report on Form 8-K on May 17, 1999 and
                     incorporated herein by reference).

   10.1              Supply Agreement dated as of May 26, 1999 by and
                     between Repligen Corporation and Amersham Pharmacia
                     Biotech AB (with certain confidential information
                     deleted) (filed herewith).

   27.1              Financial Data Schedule (filed herewith)


                                       18