SECURITIES AND EXCHANGE COMMISSION

                             Washington, D.C. 20549

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

                                    FORM 8-A

                FOR REGISTRATION OF CERTAIN CLASSES OF SECURITIES
                    PURSUANT TO SECTION 12 (b) OR (g) OF THE
                         SECURITIES EXCHANGE ACT OF 1934

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

           Delaware                                            04-2729386
   (State of incorporation                                   (IRS Employer
       or organization)                                   identification No.)

             41 Seyon Street, Bldg. 1, Suite 100, Waltham, MA 02453
               (Address of principal executive offices) (Zip Code)


If this form relates to the registration of a class of securities pursuant to
Section 12(b) of the Exchange Act and is effective pursuant to General
Instruction A.(c), please check the following box. |_|

If this form relates to the registration of a class of securities pursuant to
Section 12(g) of the Exchange Act and is effective pursuant to General
Instruction A.(c), please check the following box. |X|

        Securities to be registered pursuant to Section 12(b) of the Act:

      Title of each class                     Name of each exchange on which
      to be so registered                     each class is to be registered
      -------------------                     ------------------------------

             None                                          N/A

        Securities to be registered pursuant to Section 12(g) of the Act:

          Series A Junior Participating Preferred Stock Purchase Rights
                                (Title of Class)



Item 1. Description of Registrant's Securities to be Registered.

      On March 3, 2003, the Board of Directors of Repligen Corporation (the
"Company") declared a dividend of one preferred share purchase right (a "Right")
for each outstanding share of common stock, par value $0.01 per share (the
"Common Shares") outstanding on March 17, 2003 (the "Record Date") to the
stockholders of record on that date. Each Right entitles the registered holder
to purchase from the Company one one-thousandth of a share of Series A Junior
Participating Preferred Stock, par value $0.01 per share (the "Preferred
Shares"), of the Company, at a price of $50.00 per one one-thousandth of a
Preferred Share (the "Purchase Price"), subject to adjustment. This summary
description and terms of the Rights set forth herein are qualified in their
entirety by that certain Rights Agreement (the "Rights Agreement") between the
Company and American Stock Transfer & Trust Company (the "Rights Agent") dated
as of March 3, 2003, which is hereby incorporated herein in its entirety by
reference.

      Until the earlier to occur of (i) the close of business on the tenth
calendar day after a public announcement or disclosure of facts that a person or
group of affiliated or associated persons (an "Acquiring Person") has acquired
beneficial ownership of 15% or, in the case of a Grandfathered Stockholder (as
such term is defined in the Rights Agreement) 20% or more of the outstanding
Common Shares or (ii) the tenth business day (or such later date as may be
determined by action of the Board of Directors prior to such time as any Person
becomes an Acquiring Person) following the commencement of, or the public
announcement of an intention to make, a tender or exchange offer, the
consummation of which would result in the beneficial ownership by a person or
group of 15% or, in the case of a Grandfathered Stockholder, 20% or more of such
outstanding Common Shares (the earlier of such dates being called the
"Distribution Date"), the Rights will be evidenced, with respect to any of the
Common Share certificates outstanding as of the Record Date, by such Common
Share certificate with a copy of this Summary of Rights attached thereto.

      The Rights Agreement provides that, until the Distribution Date, the
Rights will be transferred with and only with the Common Shares. Until the
Distribution Date (or earlier redemption or expiration of the Rights), new
Common Share certificates issued after the Record Date or upon transfer or new
issuance of Common Shares will contain a notation incorporating the Rights
Agreement by reference. Until the Distribution Date (or earlier redemption or
expiration of the Rights), the surrender for transfer of any certificates for
Common Shares outstanding as of the Record Date, even without such notation or a
copy of this Summary of Rights being attached thereto, will also constitute the
transfer of the Rights associated with the Common Shares represented by such
certificate. As soon as practicable following the Distribution Date, separate
certificates evidencing the Rights ("Right Certificates") will be mailed to
holders of record of the Common Shares as of the Close of Business on the
Distribution Date and such separate Right Certificates alone will evidence the
Rights.


                               Page 2 of 7 Pages


      The Rights are not exercisable until the Distribution Date. The Rights
will expire on March 2, 2013 (the "Final Expiration Date"), unless the Final
Expiration Date is extended or unless the Rights are earlier redeemed by the
Company, in each case, as described below.

      The Purchase Price payable, and the number of Preferred Shares or other
securities or property issuable, upon exercise of the Rights are subject to
adjustment from time to time to prevent dilution (i) in the event of a stock
dividend on, or a subdivision, combination or reclassification of, the Preferred
Shares, (ii) upon the grant to holders of the Preferred Shares of certain rights
or warrants to subscribe for or purchase Preferred Shares at a price, or
securities convertible into Preferred Shares with a conversion price, less than
the then current market price of the Preferred Shares or (iii) upon the
distribution to holders of the Preferred Shares of evidences of indebtedness or
assets (excluding regular periodic cash dividends paid out of earnings or
retained earnings or dividends payable in Preferred Shares) or of subscription
rights or warrants (other than those referred to above).

      The number of outstanding Rights and the number of one one-thousandths of
a Preferred Share issuable upon exercise of each Right are also subject to
adjustment in the event of a stock split of the Common Shares or a stock
dividend on the Common Shares payable in Common Shares or subdivisions,
consolidations or combinations of the Common Shares occurring, in any such case,
prior to the Distribution Date.

      Preferred Shares purchasable upon exercise of the Rights will not be
redeemable. Each Preferred Share will be entitled to a quarterly dividend
payment of 1,000 times the dividend declared per Common Share. In the event of
liquidation, the holders of the Preferred Shares will be entitled to an
aggregate payment of 1,000 times the aggregate payment made per Common Share.
Each Preferred Share will have 1,000 votes, voting together with the Common
Shares. In the event of any merger, consolidation or other transaction in which
Common Shares are exchanged, each Preferred Share will be entitled to receive
1,000 times the amount received per Common Share. These rights are protected by
customary antidilution provisions.

      Because of the nature of the Preferred Shares' dividend, liquidation and
voting rights, the value of the one one-thousandth interest in a Preferred Share
purchasable upon exercise of each Right should approximate the value of one
Common Share.

      From and after the occurrence of an event described in Section 11(a)(ii)
of the Rights Agreement, if the Rights evidenced by the Right Certificate are or
were at any time on or after the earlier of (x) the date of such event and (y)
the Distribution Date (as such term is defined in the Rights Agreement) acquired
or beneficially owned by an Acquiring Person or an Associate or Affiliate of an
Acquiring Person (as such terms are defined in the Rights Agreement), such
Rights shall become void, and any holder of such Rights shall thereafter have no
right to exercise such Rights.


                               Page 3 of 7 Pages


      In the event that, at any time after a Person becomes an Acquiring Person,
the Company is acquired in a merger or other business combination transaction or
50% or more of its consolidated assets or earning power are sold, proper
provision will be made so that each holder of a Right will thereafter have the
right to receive, upon the exercise thereof at the then current exercise price
of the Right, that number of shares of common stock of the acquiring company
which at the time of such transaction will have a market value of two times the
exercise price of the Right. In the event that any person becomes an Acquiring
Person, proper provision shall be made so that each holder of a Right, other
than Rights beneficially owned by the Acquiring Person and its Affiliates and
Associates (which will thereafter be void), will thereafter have the right to
receive upon exercise that number of Common Shares having a market value of two
times the exercise price of the Right. If the Company does not have sufficient
Common Shares to satisfy such obligation to issue Common Shares, or if the Board
of Directors so elects, the Company shall deliver upon payment of the exercise
price of a Right an amount of cash or securities equivalent in value to the
Common Shares issuable upon exercise of a Right; provided that, if the Company
fails to meet such obligation within 30 days following the later of (x) the
first occurrence of an event triggering the right to purchase Common Shares and
(y) the date on which the Company's right to redeem the Rights expires, the
Company must deliver, upon exercise of a Right but without requiring payment of
the exercise price then in effect, Common Shares (to the extent available) and
cash equal in value to the difference between the value of the Common Shares
otherwise issuable upon the exercise of a Right and the exercise price then in
effect. The Board of Directors may extend the 30-day period described above for
up to an additional 90 days to permit the taking of action that may be necessary
to authorize sufficient additional Common Shares to permit the issuance of
Common Shares upon the exercise in full of the Rights.

      At any time after any Person becomes an Acquiring Person and prior to the
acquisition by any person or group of a majority of the outstanding Common
Shares, the Board of Directors of the Company may exchange the Rights (other
than Rights owned by such person or group which have become void), in whole or
in part, at an exchange ratio of one Common Share per Right (subject to
adjustment).

      With certain exceptions, no adjustment in the Purchase Price will be
required until cumulative adjustments require an adjustment of at least 1% in
such Purchase Price. No fractional Preferred Shares will be issued (other than
fractions which are integral multiples of one one-thousandth of a Preferred
Share, which may, at the election of the Company, be evidenced by depository
receipts) and in lieu thereof, an adjustment in cash will be made based on the
market price of the Preferred Shares on the last trading day prior to the date
of exercise.

      At any time prior to the earlier of (i) the close of business on the tenth
calendar day after the time any Person becomes an Acquiring Person (or such
later date as may be determined by the Board of Directors in its sole
discretion), or the (ii) Final Expiration Date, the Board of Directors of the
Company may redeem the Rights in whole, but not in


                               Page 4 of 7 Pages


part, at a price of $0.001 per Right (the "Redemption Price"). The redemption of
the Rights may be made effective at such time, on such basis and with such
conditions as the Board of Directors in its sole discretion may establish.
Immediately upon any redemption of the Rights, the right to exercise the Rights
will terminate and the only right of the holders of Rights will be to receive
the Redemption Price.

      The terms of the Rights may be amended by the Board of Directors of the
Company without the consent of the holders of the Rights, except that from and
after such time as any person becomes an Acquiring Person no such amendment may
adversely affect the interests of the holders of the Rights (other than the
Acquiring Person and its Affiliates and Associates).

      Until a Right is exercised, the holder thereof, as such, will have no
rights as a stockholder of the Company, including, without limitation, the right
to vote or to receive dividends.

Item 2. Exhibits

        Exhibit No.                              Exhibit
        -----------                              -------

            1           Rights Agreement, dated as of March 3, 2003, between
                        Repligen Corporation and American Stock Transfer & Trust
                        Company, which includes as Exhibit A the Form of
                        Designation of Series A Junior Participating Preferred
                        Stock, as Exhibit B the Form of Rights Certificate, and
                        as Exhibit C the Summary of Rights to Purchase Preferred
                        Shares.


                               Page 5 of 7 Pages


                                    SIGNATURE

      Pursuant to the requirements of Section 12 of the Securities Exchange Act
of 1934, the registrant has duly caused this registration statement to be signed
on its behalf by the undersigned, thereto duly authorized.

                                      REPLIGEN CORPORATION


Dated: March 4, 2003                  By: /s/ Walter C. Herlihy
                                          --------------------------------------
                                          Walter C. Herlihy,
                                          Chief Executive Officer and President,
                                          Principal Financial and Accounting
                                          Officer


                               Page 6 of 7 Pages


                                  Exhibit Index

Exhibit
 Number                           Description
 ------                           -----------

    1       Rights Agreement, dated as of March 3, 2003, between Repligen
            Corporation and American Stock Transfer & Trust Company, which
            includes as Exhibit A the Form of Designation of Series A Junior
            Participating Preferred Stock, as Exhibit B the Form of Rights
            Certificate, and as Exhibit C the Summary of Rights to Purchase
            Preferred Shares.


                               Page 7 of 7 Pages