UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                 Schedule 13G/A
                                (Amendment No. 3)

                    Under the Securities Exchange Act of 1934

                                 Repligen Corp.
                                 --------------

                     Common Stock, par value $.01 per share
                         (Title of Class of Securities)

                                February 12, 2002

                            (CUSIP Number: 759916109)

                                December 31, 2001
                                -----------------
                       Date of event which requires filing

Check the appropriate box to designate the rule pursuant to which the schedule
is filed:

                                |_| Rule 13d-1(b)
                                |X| Rule 13d-1(c)
                                |_| Rule 13d-1(d)

*The remainder of this cover page shall be filled out for a reporting person's
initial filing on this form with respect to the subject class of securities, and
for any subsequent amendment containing information which would alter
disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed
to be filled for the purpose of Section 18 of the Securities Exchange Act of
1934 ("Act") or otherwise subject to the liabilities of that section of the Act,
but shall be subject to all other provisions of the Act (however, see the Notes
to Schedule 13G).


                                       1


CUSIP No. 759916109                    13G                          Page 2 of 10
--------------------------------------------------------------------------------
1   NAME OF REPORTING PERSON
    S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

    Paramount Capital Asset Management, Inc.
--------------------------------------------------------------------------------
2   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
                                                                         (a) |_|
                                                                         (b) |_|
--------------------------------------------------------------------------------
3   SEC USE ONLY


--------------------------------------------------------------------------------
4   CITIZENSHIP OR PLACE OF ORGANIZATION

    Delaware
--------------------------------------------------------------------------------
                  5    SOLE VOTING POWER

                       None
                  --------------------------------------------------------------
  NUMBER OF       6    SHARED VOTING POWER
    SHARES
 BENEFICIALLY          2,380,650
   OWNED BY       --------------------------------------------------------------
     EACH         7    SOLE DISPOSITIVE POWER
  REPORTING
    PERSON             None
     WITH         --------------------------------------------------------------
                  8    SHARED DISPOSITIVE POWER

                       2,380,650
--------------------------------------------------------------------------------
9   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

    2,380,650
--------------------------------------------------------------------------------
10  CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*

                                                                            |_|
--------------------------------------------------------------------------------
11  PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)

    8.9%
--------------------------------------------------------------------------------
12  TYPE OF REPORTING PERSON*

    CO
--------------------------------------------------------------------------------
                     *SEE INSTRUCTIONS BEFORE FILLING OUT


                                       2


CUSIP No. 759916109                    13G                          Page 3 of 10
--------------------------------------------------------------------------------
1   NAME OF REPORTING PERSON
    S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

    Aries Domestic Fund, L.P.
--------------------------------------------------------------------------------
2   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
                                                                         (a) |_|
                                                                         (b) |_|
--------------------------------------------------------------------------------
3   SEC USE ONLY


--------------------------------------------------------------------------------
4   CITIZENSHIP OR PLACE OF ORGANIZATION

    Delaware
--------------------------------------------------------------------------------
                  5    SOLE VOTING POWER

                       None
                  --------------------------------------------------------------
  NUMBER OF       6    SHARED VOTING POWER
    SHARES
 BENEFICIALLY          545,631
   OWNED BY       --------------------------------------------------------------
     EACH         7    SOLE DISPOSITIVE POWER
  REPORTING
    PERSON             None
     WITH         --------------------------------------------------------------
                  8    SHARED DISPOSITIVE POWER

                       545,631
--------------------------------------------------------------------------------
9   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

    545,631
--------------------------------------------------------------------------------
10  CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*

                                                                            |_|
--------------------------------------------------------------------------------
11  PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)

    2.0%
--------------------------------------------------------------------------------
12  TYPE OF REPORTING PERSON*

    PN
--------------------------------------------------------------------------------
                     *SEE INSTRUCTIONS BEFORE FILLING OUT


                                       3


CUSIP No. 759916109                    13G                          Page 4 of 10
--------------------------------------------------------------------------------
1   NAME OF REPORTING PERSON
    S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

    The Aries Master Fund II
--------------------------------------------------------------------------------
2   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
                                                                         (a) |_|
                                                                         (b) |_|
--------------------------------------------------------------------------------
3   SEC USE ONLY


--------------------------------------------------------------------------------
4   CITIZENSHIP OR PLACE OF ORGANIZATION

    Cayman Islands
--------------------------------------------------------------------------------
                  5    SOLE VOTING POWER

                       None
                  --------------------------------------------------------------
  NUMBER OF       6    SHARED VOTING POWER
    SHARES
 BENEFICIALLY          1,835,019
   OWNED BY       --------------------------------------------------------------
     EACH         7    SOLE DISPOSITIVE POWER
  REPORTING
    PERSON             None
     WITH         --------------------------------------------------------------
                  8    SHARED DISPOSITIVE POWER

                       1,835,019
--------------------------------------------------------------------------------
9   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

    1,835,019
--------------------------------------------------------------------------------
10  CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*

                                                                            |_|
--------------------------------------------------------------------------------
11  PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)

    6.4%
--------------------------------------------------------------------------------
12  TYPE OF REPORTING PERSON*

    OO (see Item 2)
--------------------------------------------------------------------------------
                     *SEE INSTRUCTIONS BEFORE FILLING OUT


                                       4


CUSIP No. 759916109                    13G                          Page 5 of 10
--------------------------------------------------------------------------------
1   NAME OF REPORTING PERSON
    S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

    Lindsay A. Rosenwald, M.D.
--------------------------------------------------------------------------------
2   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
                                                                         (a) |_|
                                                                         (b) |_|
--------------------------------------------------------------------------------
3   SEC USE ONLY


--------------------------------------------------------------------------------
4   CITIZENSHIP OR PLACE OF ORGANIZATION

    United States
--------------------------------------------------------------------------------
                  5    SOLE VOTING POWER

                       33,250
                  --------------------------------------------------------------
  NUMBER OF       6    SHARED VOTING POWER
    SHARES
 BENEFICIALLY          2,380,650
   OWNED BY       --------------------------------------------------------------
     EACH         7    SOLE DISPOSITIVE POWER
  REPORTING
    PERSON             33,250
     WITH         --------------------------------------------------------------
                  8    SHARED DISPOSITIVE POWER

                       2,380,650
--------------------------------------------------------------------------------
9   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

    2,413,900
--------------------------------------------------------------------------------
10  CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*

                                                                            |_|
--------------------------------------------------------------------------------
11  PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)

    9.1%
--------------------------------------------------------------------------------
12  TYPE OF REPORTING PERSON*

    IN
--------------------------------------------------------------------------------
                     *SEE INSTRUCTIONS BEFORE FILLING OUT


                                       5


ITEM 1(a).  NAME OF ISSUER:

            Repligen Corporation.

ITEM 1(b).  ADDRESS OF ISSUER'S PRINCIPAL EXECUTIVE OFFICES:

            117 Fourth Avenue, Needham, MA 02494

ITEM 2(a)   NAME OF PERSON FILING:

            This statement is filed on behalf of Paramount Capital Asset
            Management, Inc. ("Paramount Capital"), Aries Domestic Fund, L.P.
            ("Aries Domestic"), Aries Master Fund II, a Cayman Island exempted
            company ("Aries Fund") and Lindsay A. Rosenwald, M.D. ("Dr.
            Rosenwald" and collectively, "Reporting Parties"). See attached
            Exhibit A which is a copy of their agreement in writing to file this
            statement on behalf of each of them.

            Dr. Rosenwald is an investment banker, venture capitalist, fund
            manager and sole stockholder of Paramount Capital,1 a Subchapter S
            corporation incorporated in Delaware. Paramount Capital is the
            General Partner to each of Aries Domestic,2 and Aries Domestic II,
            limited partnerships incorporated in Delaware. Paramount Capital is
            the Investment Manager to Aries Fund,3 a Cayman Islands exempted
            company.

            Dr. Rosenwald, Paramount Capital, Aries Domestic and Aries Fund and
            their respective officers, directors, general partners, investment
            managers, or trustees have not, during the five years prior to the
            date hereof, been convicted in a criminal proceeding (excluding
            traffic violations or similar misdemeanors).

ITEM 2(b)   ADDRESS OF PRINCIPAL BUSINESS OFFICE OR, IF NONE, RESIDENCE:

            Paramount Capital's, Aries Domestic's, and Dr. Rosenwald's business
            address is 787 Seventh Avenue, 48th Floor, New York, New York,
            10019. The business address for The Aries Fund is c/o Fortis Fund
            Services Cayman Limited, Grand Pavilion Commercial Center, 802 West
            Bay Road, Grand Cayman, Cayman Islands.

ITEM 2(c).  CITIZENSHIP:

            Dr. Rosenwald is a citizen of the United States of America.

ITEM 2(d).  TITLE OF CLASS OF SECURITIES:

            Common Stock, par value $0.01 per share.

----------
(1)   Please see attached Exhibit B indicating the executive officers and
      directors of Paramount Capital and providing information called for by
      Items 2-6 of this statement as to said officers and directors. Exhibit B
      is herein incorporated by reference.

(2)   Please see attached Exhibit C indicating the general partner to each of
      Aries Domestic and Aries II and the general partner's executive officers
      and directors and providing information called for by Items 2-6 of this
      statement as to said general partners, officers and directors. Exhibit C
      is herein incorporated by reference.

(3)   Please see attached Exhibit D indicating the investment manager of the
      Aries Fund and the investment manager's executive officers and directors
      and providing information called for by Items 2-6 of this statement as to
      said investment manager and officers and directors. Exhibit D is herein
      incorporated by reference.


                                       6


ITEM 2(e).  CUSIP NUMBER: 759916109

ITEM 3.     |X| CHECK THIS BOX IF THIS STATEMENT IS FILED PURSUANT TO RULE
            13d-1(c)

ITEM 4.     OWNERSHIP

            For information concerning the ownership of Common Stock of the
            Company by the Reporting Persons, see Items 5 through 9 and 11 of
            the cover pages to this schedule 13G and footnotes thereto.

ITEM 5.     OWNERSHIP OF FIVE PERCENT OR LESS OF A CLASS

            Not applicable

ITEM 6.     OWNERSHIP OF MORE THAN FIVE PERCENT ON BEHALF OF ANOTHER PERSON

            Not applicable

ITEM 7.     IDENTIFICATION AND CLASSIFICATION OF THE SUBSIDIARY WHICH ACQUIRED
            THE SECURITY BEING REPORTED ON BY THE PARENT HOLDING COMPANY

            Not Applicable

ITEM 8.     IDENTIFICATION AND CLASSIFICATION OF MEMBERS OF THE GROUP

            Not Applicable

ITEM 9.     NOTICE OF DISSOLUTION OF GROUP

            Not Applicable

ITEM 10.    CERTIFICATION

            By signing below I certify that, to the best of my knowledge and
            belief, the securities referred to above were not acquired and are
            not held for the purpose of or with the effect of changing or
            influencing the control of the issuer of the securities and were not
            acquired and are not held in connection with or as a participant in
            a transaction having that purpose and effect.

Material to be Filed as Exhibits:

Exhibit A - Copy of an Agreement between Dr. Rosenwald, Paramount Capital, Aries
            Domestic, Aries II and Aries Fund to file this Statement on Schedule
            13G on behalf of each of them.

Exhibit B - List of executive officers and directors of Paramount Capital and
            information called for by Items 2-6 of this statement relating to
            said officers and directors.

Exhibit C - List of executive officers and directors of Aries Domestic and Aries
            II and information called for by Items 2-6 of this statement
            relating to said officers and directors.

Exhibit D - List of executive officers and directors of Aries Fund and
            information called for by Items 2-6 of this statement relating to
            said officers and directors.


                                       7


                                   SIGNATURES

After reasonable inquiry and to the best of my knowledge and belief, I certify
that the information set forth in this statement is true, complete and correct.

                                    PARAMOUNT CAPITAL ASSET MANAGEMENT, INC.

Dated: February 12, 2002            By /s/ Lindsay A. Rosenwald, M.D.
New York, NY                           -----------------------------------------
                                    Name:  Lindsay A. Rosenwald, M.D.
                                    Title: Chairman


                                    ARIES DOMESTIC FUND, L.P.
                                    By Paramount Capital Asset Management, Inc.,
                                    General Partner

Dated: February 12, 2002
New York, NY                        By /s/ Lindsay A. Rosenwald, M.D.
                                       -----------------------------------------
                                    Name:  Lindsay A. Rosenwald, M.D.
                                    Title: Chairman


                                    THE ARIES MASTER FUND II
                                    By Paramount Capital Asset Management, Inc.
                                    Investment Manager

Dated: February 12, 2002
New York, NY                        By /s/ Lindsay A. Rosenwald, M.D.
                                       -----------------------------------------
                                    Name:  Lindsay A. Rosenwald, M.D.
                                    Title: Chairman

Dated: February 12, 2002
New York, NY                        By /s/ Lindsay A. Rosenwald, M.D.
                                       -----------------------------------------
                                    Name:  Lindsay A. Rosenwald, M.D.


                                       8


                                    EXHIBIT A

                                    AGREEMENT

                          JOINT FILING OF SCHEDULE 13G

The undersigned hereby agrees to jointly prepare and file with regulatory
authorities a Schedule 13G and any future amendments thereto reporting each of
the undersigned's ownership of securities of Repligen Corp.and hereby affirm
that such Schedule 13G is being filed on behalf of each of the undersigned.

                                    PARAMOUNT CAPITAL ASSET MANAGEMENT, INC.

Dated: February 10, 2002
New York, NY                        By /s/ Lindsay A. Rosenwald, M.D.
                                       -----------------------------------------
                                    Name:  Lindsay A. Rosenwald, M.D.
                                    Title: Chairman


                                    ARIES DOMESTIC FUND, L.P.
                                    By Paramount Capital Asset Management, Inc.,
                                    General Partner

Dated: February 10, 2002
New York, NY                        By /s/ Lindsay A. Rosenwald, M.D.
                                       -----------------------------------------
                                    Name:  Lindsay A. Rosenwald, M.D.
                                    Title: Chairman


                                    THE ARIES DOMESTIC FUND II, L.P.
                                    By Paramount Capital Asset Management, Inc.
                                    General Partner

Dated: February 10, 2002
New York, NY                        By /s/ Lindsay A. Rosenwald, M.D.
                                       -----------------------------------------
                                    Name:  Lindsay A. Rosenwald, M.D.
                                    Title: Chairman


                                    THE ARIES MASTER FUND II
                                    By Paramount Capital Asset Management, Inc.
                                    Investment Manager

Dated: February 10, 2002
New York, NY                        By /s/ Lindsay A. Rosenwald, M.D.
                                       -----------------------------------------
                                    Name:  Lindsay A. Rosenwald, M.D.
                                    Title: Chairman

Dated: February 10, 2002
New York, NY                        By /s/ Lindsay A. Rosenwald, M.D.
                                       -----------------------------------------
                                    Name:  Lindsay A. Rosenwald, M.D.


                                       9


                                    EXHIBIT B

The name and principal occupation or employment, which in each instance is with
Paramount Capital Asset Management, Inc. ("Paramount Capital") located at 787
Seventh Avenue, 48th Floor, New York, New York, 10019, of each executive officer
and director of Paramount Capital is as follows:

NAME                                    PRINCIPAL OCCUPATION
----                                    OR EMPLOYMENT
                                        -------------

Lindsay A. Rosenwald, M.D.              Chairman of the Board and sole
                                        shareholder, Paramount Capital Asset
                                        Management, Inc., Paramount Capital
                                        Investments LLC and Paramount Capital,
                                        Inc.

Mark C. Rogers, M.D.                    President of Paramount Capital Asset
                                        Management, Inc., Paramount Capital
                                        Investments, LLC, and Paramount Capital,
                                        Inc.

Peter Morgan Kash                       Director of Paramount Capital Asset
                                        Management, Inc., Senior Managing
                                        Director, Paramount Capital, Inc.

Dr. Yuichi Iwaki                        Director of Paramount Capital Asset
                                        Management, Inc., Professor, University
                                        of Southern California School of
                                        Medicine

Item 2.

During the five years prior to the date hereof, the above person (to the best of
Paramount Capital's knowledge) has not been convicted in a criminal proceeding
(excluding traffic violations or similar misdemeanors). Additionally, during the
five years prior to the date hereof, the above person (to the best of Paramount
Capital's knowledge) has not been a party to a civil proceeding, of a judicial
or administrative body of competent jurisdiction, as a result of which such
person was or is subject to a judgment, decree or final order enjoining future
violations of, or prohibiting or mandating activities subject to, Federal or
State securities laws or finding any violation with respect to such laws.

Items 3-6.

Please refer to Items 3-6 herein reporting the beneficial ownership.


                                       10


                                    EXHIBIT C

The name and principal occupation or employment of the General Partner of Aries
Domestic and Aries II, which are located at 787 Seventh Avenue, 48th Floor, New
York, New York, 10019, is as follows:

NAME                                       PRINCIPAL OCCUPATION
----                                       OR EMPLOYMENT
                                           -------------

Paramount Capital Asset Management, Inc    General Partner; Investment Manager

Exhibit B is hereby incorporated by reference.

Item 2.

During the five years prior to the date hereof, the above person (to the best of
Aries Domestic's and Aries II's knowledge) has not been convicted in a criminal
proceeding (excluding traffic violations or similar misdemeanors). Additionally,
during the five years prior to the date hereof, the above person (to the best of
Aries Domestic's and Aries II's knowledge) has not been a party to a civil
proceeding, of a judicial or administrative body of competent jurisdiction, as a
result of which such person was or is subject to a judgment, decree or final
order enjoining future violations of, or prohibiting or mandating activities
subject to, Federal or State securities laws or finding any violation with
respect to such laws.

Items 3-6.

Please refer to Items 3-6 herein reporting the beneficial ownership.


                                       11


                                    EXHIBIT D

The name and principal occupation or employment, which in the case of Paramount
Capital Asset Management, Inc. is located at 787 Seventh Avenue, 48th Floor, New
York, New York, 10019, of each executive officer and director of Aries Fund is
as follows:

PRINCIPAL OCCUPATION                       NAME
OR EMPLOYMENT                              ----
-------------

Paramount Capital Asset Management, Inc.   Investment Manager

Fortis Fund Services Cayman Limited        Administrator
Grand Pavilion Commercial Center
802 West Bay Road
Grand Cayman, Cayman Islands

Exhibit B is hereby incorporated by reference.

Item 2.

During the five years prior to the date hereof, the above person (to the best of
Aries Fund's knowledge) has not been convicted in a criminal proceeding
(excluding traffic violations or similar misdemeanors). Additionally, during the
five years prior to the date hereof, the above person (to the best of Aries
Fund's knowledge) has not been a party to a civil proceeding, of a judicial or
administrative body of competent jurisdiction, as a result of which such person
was or is subject to a judgment, decree or final order enjoining future
violations of, or prohibiting or mandating activities subject to, Federal or
State securities laws or finding any violation with respect to such laws.

Items 3-6.

Please refer to Items 3-6 herein reporting the beneficial ownership.


                                       12