FORM 3

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 INITIAL STATEMENT OF BENEFICIAL OWNERSHIP OF SECURITIES Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

OMB APPROVAL OMB Number: 3235-0104 Expires: January 31, 2005 Estimated average burden hours per response... 0.5

(Print or Type Responses)

1. Name and Address of Reporting Person *   BVF PARTNERS L P/IL 2. Date of Event Requiring Statement (Month/Day/Year) 05/15/2009 3. Issuer Name and Ticker or Trading Symbol REPLIGEN CORP [RGEN] (Last) (First) (Middle) 900 N. MICHIGAN AVENUE, SUITE 1100 4. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director __X__ 10% Owner _____ Officer (give title below) __X__ Other (specify below) Group Member 5. If Amendment, Date Original Filed(Month/Day/Year) (Street) CHICAGO, IL 60611 6. Individual or Joint/Group Filing(Check Applicable Line) ___ Form filed by One Reporting Person _X_ Form filed by More than One Reporting Person (City) (State) (Zip) Table I - Non-Derivative Securities Beneficially Owned

1.Title of Security (Instr. 4) 2. Amount of Securities Beneficially Owned (Instr. 4) 3. Ownership Form: Direct (D) or Indirect (I) (Instr. 5) 4. Nature of Indirect Beneficial Ownership (Instr. 5) Common Stock 825,750 D (1)   Common Stock 534,400 D (2)   Common Stock 1,856,700 D (3)   Common Stock 3,216,850 I (4) General partner and manager of entities with direct ownership. Common Stock 3,216,850 I (5) General partner of entity with indirect ownership. Common Stock 3,216,850 I (6) Sole shareholder and sole director of entity with indirect ownership.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.			SEC 1473 (7-02)
	Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB control number.

Table II - Derivative Securities Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)

1. Title of Derivative Security (Instr. 4)	2. Date Exercisable and Expiration Date (Month/Day/Year)		3. Title and Amount of Securities Underlying Derivative Security (Instr. 4)		4. Conversion or Exercise Price of Derivative Security	5. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 5)	6. Nature of Indirect Beneficial Ownership (Instr. 5)
	Date Exercisable	Expiration Date	Title	Amount or Number of Shares

Reporting Owners

Reporting Owner Name / Address	Relationships
	Director	10% Owner	Officer	Other
BVF PARTNERS L P/IL 900 N. MICHIGAN AVENUE SUITE 1100 CHICAGO, IL 60611	Â	  X	Â	Group Member
BIOTECHNOLOGY VALUE FUND L P 900 N. MICHIGAN AVENUE, SUITE 1100 CHICAGO, IL 60611	Â	  X	Â	Group Member
BIOTECHNOLOGY VALUE FUND II LP C/O GROSVENOR CAPITAL MANAGEMENT 900 N. MICHIGAN AVENUE, SUITE 1100 CHICAGO, IL 60611	Â	  X	Â	Group Member
BVF INVESTMENTS LLC C/O GROSVENOR CAPITAL MANAGEMENT LP 900 N. MICHIGAN AVENUE, SUITE 1100 CHICAGO, IL 60611	Â	  X	Â	Group Member
BVF INC/IL C/O GROSVENOR CAPITAL MANAGEMENT 900 N. MICHIGAN AVENUE, SUITE 1100 CHICAGO, IL 60611	Â	  X	Â	Group Member
LAMPERT MARK N 2415 GREEN ST SAN FRANCISCO, CA 94123	Â	  X	Â	Group Member

Signatures

BVF PARTNERS L.P., By: BVF Inc., its General Partner, By: /s/ Mark N. Lampert, President		05/20/2009
**Signature of Reporting Person		Date
BIOTECHNOLOGY VALUE FUND, L.P., By: BVF Partners L.P., its General Partner, By: BVF Inc., its General Partner, By: /s/ Mark N. Lampert, President		05/20/2009
**Signature of Reporting Person		Date
BIOTECHNOLOGY VALUE FUND II, L.P., By: BVF Partners L.P., its General Partner, By: BVF Inc., its General Partner, By: /s/ Mark N. Lampert, President		05/20/2009
**Signature of Reporting Person		Date
BVF INVESTMENTS, L.L.C., By: BVF Partners L.P., its Manager, By: BVF Inc., its General Partner, By: /s/ Mark N. Lampert, President		05/20/2009
**Signature of Reporting Person		Date
BVF INC., By: /s/ Mark N. Lampert, President		05/20/2009
**Signature of Reporting Person		Date
Mark N. Lampert, By: /s/ Mark N. Lampert		05/20/2009
**Signature of Reporting Person		Date

Explanation of Responses:

*	If the form is filed by more than one reporting person, see Instruction 5(b)(v).
**	Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
(1)	The shares of Common Stock are directly beneficially owned by Biotechnology Value Fund, L.P. ("BVF"), a Delaware limited partnership.
(2)	The shares of Common Stock are directly beneficially owned by Biotechnology Value Fund II, L.P. ("BVF2"), a Delaware limited partnership.
(3)	The shares of Common Stock are directly beneficially owned by BVF Investments, LLC ("BVFLLC"), a Delaware limited liability company. Pursuant to the operating agreement of BVFLLC, BVF Partners, L.P., a Delaware limited partnership ("Partners") is authorized, among other things, to invest the contributed capital of Samana Capital, L.P., the majority member of BVFLLC, in the shares of Common Stock and other securities and to vote, exercise or convert and dispose of such securities and is entitled to receive fees based on assets under management and, subject to certain exceptions, allocations based on realized and unrealized gains on such assets.
(4)	The shares of Common Stock are indirectly beneficially owned by Partners. Partners is the general partner of BVF and BVF2 and is the manager of BVFLLC.
(5)	The shares of Common Stock are indirectly beneficially owned by BVF Inc., a Delaware corporation ("BVF Inc."). BVF Inc. is the general partner of Partners.
(6)	Mark N. Lampert is the sole shareholder, sole director, and an officer of BVF Inc.

Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, See Instruction 6 for procedure.

Potential persons who are to respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB number.