SECURITIES AND EXCHANGE COMMISSION
                             Washington, D. C. 20549

                                  SCHEDULE 13G

                    Under the Securities Exchange Act of 1934
                               (Amendment No. __)

                              REPLIGEN CORPORATION
                                (Name of Issuer)

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

                                    759916109
                                 (CUSIP Number)

                                October 27, 2004
             (Date of Event which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this 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 the
disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the Securities Exchange Act of
1934 (the "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).



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CUSIP No. 759916109                                            Page 2 of 8 Pages
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1.   NAME OF REPORTING PERSONS
     S.S. OR I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS

     IB PARTNERS MANAGEMENT, INC., IRS number 74-3092655, ,as general partner on
     behalf of the following limited partnerships and on behalf of managed 
     accounts:

     InvestBio Partners, L.P., IRS number 74-3092654, and InvestBio Short Term 
     Opportunity Fund, L.P., IRS number 20-1477796.
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2.   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP

                                                                 (a)  |_|
                                                                 (b)  |_|
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3.   SEC USE ONLY


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4.   CITIZENSHIP OR PLACE OF ORGANIZATION

     Delaware
--------------------------------------------------------------------------------
  NUMBER OF    5.   SOLE VOTING POWER

   SHARES           1,583,878
               -----------------------------------------------------------------
BENEFICIALLY   6.   SHARED VOTING POWER

  OWNED BY          0
               -----------------------------------------------------------------
    EACH       7.   SOLE DISPOSITIVE POWER
     
  REPORTING         1,583,878
               -----------------------------------------------------------------
   PERSON      8.   SHARED DISPOSITIVE POWER

    WITH            0
--------------------------------------------------------------------------------
9.   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

     1,583,878
--------------------------------------------------------------------------------
10.  CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*

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

     5.3%
--------------------------------------------------------------------------------
12.  TYPE OF REPORTING PERSON*

     IB Partners Management, Inc.-  CO                                      
     InvestBio Partners, L.P. and InvestBio Short Term Opportunity Fund-  PN
--------------------------------------------------------------------------------


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CUSIP No. 759916109                                            Page 3 of 8 Pages
-------------------                                            -----------------

Item 1(a).        Name of Issuer:

                  Repligen Corporation

Item 1(b).        Address of Issuer's Principal Executive Offices:

                  41 Seyon Street, Bldg. 1, Suite 100, Waltham, MA  02453

Item 2(a).        Name of Person Filing:

                  (i)   IB Partners Management, Inc.
                  (ii)  InvestBio Partners, L.P.
                  (iii) InvestBio Short Term Opportunity Fund, L.P.

Item 2(b).        Address of Principal Business Office or, if None, Residence:

                  500 Fifth Avenue, 56th Fl., New York, NY 10110

Item 2(c).        Citizenship:

                  Not Applicable.

Item 2(d).        Title of Class of Securities:

                  Common Stock, $.01 par value.

Item 2(e).        CUSIP Number:

                  759916109

Item 3.           If this statement is filed pursuant to Rule 13d-1(b), or 
                  13d-2(b) or (c), check whether the person filing is a:

            (a)   |_| Broker or Dealer registered under Section 15 of the Act.

            (b)   |_| Bank as defined in Section 3(a)(6) of the Act.

            (c)   |_| Insurance Company as defined in Section 3(a)(19) of the
                      Act.

            (d)   |_| Investment Company registered under Section 8 of the
                      Investment Company Act.

            (e)   |_| Investment Adviser registered in accordance with Rule
                      13d-1(b)(1)(ii)(E).


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CUSIP No. 759916109                                            Page 4 of 8 Pages
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            (f)   |_| Employee Benefit Plan or Endowment Fund in accordance with
                      Rule 13d-1(b)(1)(ii)(F).

            (g)   |_| Parent Holding Company or control person in accordance
                      with Rule 13d-1(b)(1)(ii)(G).

            (h)   |_| A savings association as defined in Section 3(b) of the
                      Federal Deposit Insurance Act.

            (i)   |_| A church plan that is excluded from the definition of an
                      investment company under Section 3(c) (14) of the 
                      Investment Company Act of 1940.

            (j)   |_| Group, in accordance with Rule 13d-1(b)(1)(ii)(J).

                      If this Statement is filed pursuant to Rule 13d-1(c), 
                      check this box [X].

Item 4.           Ownership.

                  (a)   Amount beneficially owned: 1,583,878

                  (b)   Percent of Class: 5.3%

                  (c)   Number of Shares as to which the person has:

                        (i)   Sole power to vote or to direct the vote:
                              1,583,878

                        (ii)  Shared power to vote or to direct the vote: 0

                        (iii) Sole power to dispose or to direct the disposition
                              of: 1,583,878

                        (iv)  Shared Power to dispose or to direct the
                              disposition of: 0

Item 5.           Ownership of Five Percent or Less of a Class.

            If this statement is being filed to report the fact that as of the
date hereof the reporting person has ceased to be the beneficial owner of more
than five percent of the class of securities, check the following. |_|.

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.


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CUSIP No. 759916109                                            Page 5 of 8 Pages
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Item 8.           Identification and Classification of Members of the Group.

                  See Exhibit A

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 any transaction having
                  that purpose or effect.

                                    SIGNATURE

Date: November 5, 2004                  IB PARTNERS MANAGEMENT, INC.,

                                        By: /s/ Scott L. Mathis
                                            ------------------------------
                                            Name: Scott L. Mathis
                                            Title: President


                                        INVESTBIO PARTNERS, L.P. and
                                        INVESTBIO SHORT TERM
                                            OPPORTUNITY FUND, L.P.

                                        By: IB PARTNERS MANAGEMENT, INC.,
                                               General Partner

                                        By: /s/ Scott L. Mathis
                                            ------------------------------
                                            Name: Scott L. Mathis
                                            Title: President


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                                    EXHIBIT A

                IDENTIFICATION AND CLASSIFICATION OF JOINT FILERS

      Pursuant to Rule 13d-1(k)(1) under the Securities Exchange Act of 1934,
the reporting persons making this joint filing are identified and classified as
follows:



         Name                                 Classification
         ----                                 --------------
                                           
IB Partners Management, Inc.                  Corporate general partner of InvestBio 
                                              Partners, L.P. and InvestBio Short Term 
                                              Opportunity Fund, L.P.

InvestBio Partners, L.P.                      Partnership

InvestBio Short Term Opportunity Fund, L.P.   Partnership



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                                                                       EXHIBIT B

               JOINT FILING AGREEMENT PURSUANT TO RULE 13d-1(k)(1)

      This agreement is made pursuant to Rule 13d-1(k)(1) under the Securities
and Exchange Act of 1934 (the "Act") by and among the parties listed below, each
referred to herein as a "Joint Filer." The Joint Filers agree that a statement
of beneficial ownership as required by Sections 13(g) or 13(d) of the Act and
the Rules thereunder maybe filed on each of their behalf on Schedule 13G or 13D,
as appropriate, and that said joint filing may thereafter be amended by further
joint filings. The Joint Filers state that they each satisfy the requirements
for making a joint filing under Rule 13d-1(k) (1).

                                        IB PARTNERS MANAGEMENT, INC.,

                                        By: /s/ Scott L. Mathis
                                            ------------------------------
                                            Name: Scott L. Mathis
                                            Title: President


                                        INVESTBIO PARTNERS, L.P. and
                                        INVESTBIO SHORT TERM
                                            OPPORTUNITY FUND, L.P.

                                        By: IB PARTNERS MANAGEMENT, INC.,
                                               General Partner

                                        By: /s/ Scott L. Mathis
                                            ------------------------------
                                            Name: Scott L. Mathis
                                            Title: President


                                       7


                                                                       EXHIBIT C

                       DISCLAIMER OF BENEFICIAL OWNERSHIP

Repligen Corporation
Common Stock
1,583,878 shares

IB Partners Management, Inc. disclaims beneficial ownership as to all shares
beneficially owned for Section 13(g) filing purposes by InvestBio Partners,
L.P., InvestBio Short Term Opportunity Fund, L.P. and managed accounts.


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