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

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(Print or Type Responses)

1. Name and Address of Reporting Person *   PROSPECT VENTURE PARTNERS II LP 2. Date of Event Requiring Statement (Month/Day/Year) 09/12/2006 3. Issuer Name and Ticker or Trading Symbol INFINITY PHARMACEUTICALS, INC. [INFI] (Last) (First) (Middle) C/O PROSPECT VENTURE PARTNERS, 435 TASSO STREET, SUITE 200 4. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director __X__ 10% Owner _____ Officer (give title below) _____ Other (specify below) 5. If Amendment, Date Original Filed(Month/Day/Year) (Street) PALO ALTO, CA 94301 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 107,750 (1) (8) I By Prospect Venture Partners II, L.P. (2) Common Stock 510,575 (1) (9) I By Prospect Venture Partners II, L.P. (3) Common Stock 799,152 (1) (10) I By Prospect Venture Partners II, L.P. (4) Common Stock 107,750 (1) (11) I By Prospect Venture Partners, L.P. (5) Common Stock 510,575 (1) (12) I By Prospect Venture Partners, L.P. (6) Common Stock 66,596 (1) (13) I By Prospect Venture Partners, L.P. (7)

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.			SEC 1473 (7-02)
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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
PROSPECT VENTURE PARTNERS II LP C/O PROSPECT VENTURE PARTNERS 435 TASSO STREET, SUITE 200 PALO ALTO, CA 94301	Â	  X	Â	Â
PROSPECT VENTURE PARTNERS LP C/O PROSPECT VENTURE PARTNERS 435 TASSO STREET, SUITE 200 PALO ALTO, CA 94301	Â	  X	Â	Â
PROSPECT MANAGEMENT CO II LLC C/O PROSPECT VENTURE PARTNERS 435 TASSO STREET, SUITE 200 PALO ALTO, CA 94301	Â	  X	Â	Â
PROSPECT MANAGEMENT CO LLC C/O PROSPECT VENTURE PARTNERS 435 TASSO STREET, SUITE 200 PALO ALTO, CA 94301	Â	  X	Â	Â

Signatures

/s/ David Schnell, Managing Director of Prospect Management Co. II, L.L.C.		09/22/2006
**Signature of Reporting Person		Date
/s/ David Schnell, Managing Director of Prospect Management Co., L.L.C.		09/22/2006
**Signature of Reporting Person		Date
/s/ David Schnell, Managing Director of Prospect Management Co. II, L.L.C., which serves as the sole general partner of Prospect Venture Partners II, L.P.		09/22/2006
**Signature of Reporting Person		Date
/s/ David Schnell, Managing Director of Prospect Management Co., L.L.C., which serves as the sole general partner of Prospect Venture Partners, L.P.		09/22/2006
**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)	Reflects a 1:4 reverse stock split, which became effective September 12, 2006.
(2)	The shares are owned by Prospect Venture Partners II, L.P. ("PVP II"). Prospect Management Co. II, L.L.C. ("PMC II") serves as the sole general partner of PVP II. James B. Tananbaum, M.D., Alexander E. Barkas, Ph.D., David Schnell, M.D., and Russell C. Hirsch, M.D., Ph.D. are the Managing Directors of PMC II. Each reporting person disclaims beneficial ownership of the shares reported herein, except to the extent of its pecuniary interest. Dr. Tananbaum, Dr. Barkas and Dr. Schnell have filed separate Section 16 reports relating to their holdings in the Issuer.
(3)	The shares are owned by PVP II.
(4)	The shares are owned by PVP II.
(5)	The shares are owned by Prospect Venture Partners, L.P. ("PVP"). Prospect Management Co., L.L.C. ("PMC") serves as the sole general partner of PVP. Dr. Barkas and Dr. Schnell are the Managing Directors of PMC. Each reporting person disclaims beneficial ownership of the shares reported herein, except to the extent of its pecuniary interest.
(6)	The shares are owned by PVP.
(7)	The shares are owned by PVP.
(8)	Received in exchange for 487,500 shares of Infinity Discovery, Inc. ("IPI") Common Stock in connection with the merger of IPI into Infinity Pharmaceuticals, Inc. (the "Merger") based on a conversion ratio of 0.88411.
(9)	Received in exchange for 2,600,000 shares of IPI Series A Preferred Stock in connection with the Merger based on a conversion ratio of 0.78550. The Series A Preferred Stock had no expiration date.
(10)	Received in exchange for 3,200,000 shares of IPI Series B Preferred Stock in connection with the Merger based on a conversion ratio of 0.99894. The Series B Preferred Stock had no expiration date.
(11)	Received in exchange for 487,500 shares of IPI Common Stock in connection with the Merger based on a conversion ratio of 0.88411.
(12)	Received in exchange for 2,600,000 shares of IPI Series A Preferred Stock in connection with the Merger based on a conversion ratio of 0.78550. The Series A Preferred Stock had no expiration date.
(13)	Received in exchange for 266,667 shares of IPI Series B Preferred Stock in connection with the Merger based on a conversion ratio of 0.99894. The Series B Preferred Stock had no expiration date.

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