FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN 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 * GAY ROBERT C   2. Issuer Name and Ticker or Trading Symbol MERRIMACK PHARMACEUTICALS INC [MACK] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) __X__ Director _____ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) C/O MERRIMACK PHARMACEUTICALS, INC., ONE KENDALL SQUARE, SUITE B7201 3. Date of Earliest Transaction (Month/Day/Year) 04/03/2012 (Street) CAMBRIDGE, MA 02139 4. If Amendment, Date Original Filed(Month/Day/Year) 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 Acquired, Disposed of, or Beneficially Owned

1.Title of Security (Instr. 3) 2. Transaction Date (Month/Day/Year) 2A. Deemed Execution Date, if any (Month/Day/Year) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 04/03/2012   C   175,316 A (1) 175,316 D   Common Stock 04/03/2012   C   142,857 A (2) 318,173 D   Common Stock 04/03/2012   C   175,316 A (1) 493,489 I See footnote (3) Common Stock 04/03/2012   C   142,857 A (2) 636,346 I See footnote (3)

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

1. Title of Derivative Security (Instr. 3)	2. Conversion or Exercise Price of Derivative Security	3. Transaction Date (Month/Day/Year)	3A. Deemed Execution Date, if any (Month/Day/Year)	4. Transaction Code (Instr. 8)		5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5)		6. Date Exercisable and Expiration Date (Month/Day/Year)		7. Title and Amount of Underlying Securities (Instr. 3 and 4)		8. Price of Derivative Security (Instr. 5)	9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4)	10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4)	11. Nature of Indirect Beneficial Ownership (Instr. 4)
				Code	V	(A)	(D)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares
Series B Convertible Preferred Stock	(1)	04/03/2012		C			113,586	(1)	(1)	Common Stock	175,316 (1)	$ 0	0	D
Series D Convertible Preferred Stock	(2)	04/03/2012		C			142,857	(2)	(2)	Common Stock	142,857	$ 0	0	D
Series B Convertible Preferred Stock	(1)	04/03/2012		C			113,586	(1)	(1)	Common Stock	175,316 (1)	$ 0	0	I	See footnote (3)
Series D Convertible Preferred Stock	(2)	04/03/2012		C			142,857	(2)	(2)	Common Stock	142,857	$ 0	0	I	See footnote (3)

Reporting Owners

Reporting Owner Name / Address	Relationships
	Director	10% Owner	Officer	Other
GAY ROBERT C C/O MERRIMACK PHARMACEUTICALS, INC. ONE KENDALL SQUARE, SUITE B7201 CAMBRIDGE, MA 02139	X

Signatures

/s/ Jeffrey A. Munsie, attorney-in-fact		04/05/2012
**Signature of Reporting Person		Date

Explanation of Responses:

*	If the form is filed by more than one reporting person, see Instruction 4(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 automatically converted into approximately 1.5435 shares of Common Stock upon the closing of the issuer's initial public offering and had no expiration date.
(2)	Each share of Series D Convertible Preferred Stock automatically converted into one share of Common Stock upon the closing of the issuer's initial public offering and had no expiration date.
(3)	These shares are held by the Robert C. Gay 1998 Family Trust, of which Dr. Gay is the grantor. Dr. Gay disclaims beneficial ownership of these shares, except to the extent of his pecuniary interest therein, and the inclusion of these shares in this report shall not be deemed an admission of beneficial ownership of any such shares for purposes of Section 16 of the Securities Exchange Act of 1934, as amended, or for any other purpose.

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