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 *   Mulroy Robert J. 2. Date of Event Requiring Statement (Month/Day/Year) 01/31/2012 3. Issuer Name and Ticker or Trading Symbol MERRIMACK PHARMACEUTICALS INC [MACK] (Last) (First) (Middle) C/O MERRIMACK PHARMACEUTICALS, INC., ONE KENDALL SQUARE, SUITE B7201 4. Relationship of Reporting Person(s) to Issuer (Check all applicable) __X__ Director _____ 10% Owner __X__ Officer (give title below) _____ Other (specify below) President and CEO 5. If Amendment, Date Original Filed(Month/Day/Year) (Street) CAMBRIDGE, MA 02139 6. Individual or Joint/Group Filing(Check Applicable Line) _X_ Form filed by One Reporting Person ___ 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 474,603 D Â

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
Series B Convertible Preferred Stock	  (1)	  (1)	Common Stock	26,173	$ (1)	D	Â
Series C Convertible Preferred Stock	  (2)	  (2)	Common Stock	29,019	$ (2)	D	Â
Series D Convertible Preferred Stock	  (3)	  (3)	Common Stock	57,143	$ (3)	I	By Wife
Series E Convertible Preferred Stock	  (4)	  (4)	Common Stock	4,657	$ (4)	I	By Wife
Stock Option (right to buy)	  (5)	08/02/2012	Common Stock	75,000	$ 2.19	D	Â
Stock Option (right to buy)	  (5)	05/08/2013	Common Stock	50,000	$ 2.19	D	Â
Stock Option (right to buy)	  (5)	08/30/2014	Common Stock	158,048	$ 1.25	D	Â
Stock Option (right to buy)	  (5)	08/30/2014	Common Stock	141,952	$ 1.25	D	Â
Stock Option (right to buy)	  (5)	02/02/2015	Common Stock	25,837	$ 1.25	D	Â
Stock Option (right to buy)	  (5)	02/02/2015	Common Stock	224,163	$ 1.25	D	Â
Stock Option (right to buy)	  (5)	08/03/2015	Common Stock	43,247	$ 1.71	D	Â
Stock Option (right to buy)	  (5)	08/03/2015	Common Stock	456,753	$ 1.71	D	Â
Stock Option (right to buy)	  (5)	01/23/2017	Common Stock	52,985	$ 2.47	D	Â
Stock Option (right to buy)	  (5)	01/23/2017	Common Stock	97,015	$ 2.47	D	Â
Stock Option (right to buy)	  (5)	10/04/2017	Common Stock	26,689	$ 2.59	D	Â
Stock Option (right to buy)	  (5)	10/04/2017	Common Stock	248,311	$ 2.59	D	Â
Stock Option (right to buy)	  (6)	11/04/2019	Common Stock	775,000	$ 2.12	D	Â
Stock Option (right to buy)	  (7)	05/02/2021	Common Stock	50,000	$ 5.54	D	Â

Reporting Owners

Reporting Owner Name / Address	Relationships
	Director	10% Owner	Officer	Other
Mulroy Robert J. C/O MERRIMACK PHARMACEUTICALS, INC. ONE KENDALL SQUARE, SUITE B7201 CAMBRIDGE, MA 02139	  X	Â	  President and CEO	Â

Signatures

/s/ Jeffrey A. Munsie, attorney-in-fact		02/01/2012
**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)	Each share of Series B Convertible Preferred Stock will automatically convert into approximately 1.5435 shares of Common Stock upon the closing of the issuer's initial public offering. The shares have no expiration date.
(2)	Each share of Series C Convertible Preferred Stock will automatically convert into one share of Common Stock upon the closing of the issuer's initial public offering. The shares have no expiration date.
(3)	Each share of Series D Convertible Preferred Stock will automatically convert into one share of Common Stock upon the closing of the issuer's initial public offering. The shares have no expiration date.
(4)	Each share of Series E Convertible Preferred Stock will automatically convert into one share of Common Stock upon the closing of the issuer's initial public offering. The shares have no expiration date.
(5)	This option is fully vested.
(6)	This option vested as to 1/12th of the shares on each of November 5, 2009 and February 1, 2010 and vests in equal quarterly installments thereafter until August 1, 2012.
(7)	This option vested as to 1/12th of the shares on August 1, 2011 and vests in equal quarterly installments thereafter until May 1, 2014.

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.