SCHEDULE 13G

Amendment No. 0 
MERRIMACK PHARMACEUTICALS INC 
Common Stock 
Cusip #590328100 


Cusip #590328100 
Item 1:	Reporting Person - FMR LLC
Item 4:	Delaware
Item 5:	22,700 
Item 6:	0 
Item 7:	9,814,929 
Item 8:	0 
Item 9:	9,814,929 
Item 11:	10.623% 
Item 12:	    HC


Cusip #590328100  
Item 1:	Reporting Person - Edward C. Johnson 3d 
Item 4:	United States of America
Item 5:	0 
Item 6:	0 
Item 7:	9,814,929 
Item 8:	0 
Item 9:	9,814,929 
Item 11:	10.623% 
Item 12:	IN 




	SCHEDULE 13G - TO BE INCLUDED IN 
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)



Item 1(a).	Name of Issuer:	

		MERRIMACK PHARMACEUTICALS 
INC 

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

		One Kendall Square
		Ste 87201 
		Cambridge, MA  02139
		USA  

Item 2(a).	Name of Person Filing: 

		FMR LLC

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

		82 Devonshire Street, Boston, 
Massachusetts  02109

Item 2(c).	Citizenship:

		Not applicable

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

		Common Stock 

Item 2(e).	CUSIP Number:  

		590328100 

Item 3.	This statement is filed pursuant to Rule 13d-1(b) or 13d-2(b) 
and the person filing, FMR LLC, is a parent holding company 
in accordance with Section 240.13d-1(b)(ii)(G).  (Note:  See 
Item 7).

Item 4.	Ownership

	(a)	Amount Beneficially Owned:	9,814,929 

	(b)	Percent of Class:	10.623% 

	(c)	Number of shares as to which such 
person has:  

	(i)	sole power to vote or to direct 
the vote:	22,700 

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

	(iii)	sole power to dispose or to 
direct the disposition of:	9,814,929 

	(iv)	shared power to dispose or to 
direct the disposition of:	0 



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

	Not applicable.

Item 6.	Ownership of More than Five Percent on Behalf of Another 
Person.

	Various persons have the right to receive or the power to 
direct the receipt of dividends from, or the proceeds from the 
sale of, the Common Stock of MERRIMACK 
PHARMACEUTICALS INC.  No one person's interest in the 
Common Stock of MERRIMACK PHARMACEUTICALS 
INC is more than five percent of the total outstanding 
Common Stock.

Item 7.	Identification and Classification of the Subsidiary Which 
Acquired the Security Being Reported on By the Parent 
Holding Company.

	See attached Exhibit A. 

Item 8.	Identification and Classification of Members of 
the Group.

	Not applicable.  See attached 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 acquired in the 
ordinary course of business and were not acquired for the 
purpose of and do not have the effect of changing or 
influencing the control of the issuer of such securities and 
were not acquired in connection with or as a participant in any 
transaction having such purpose or effect.     

Signature

	After reasonable inquiry and to the best of my knowledge and 
belief, I certify that the information set forth in this statement 
is true, complete and correct. 

April 09, 2012 			
Date

 /s/ Scott C. Goebel			
Signature

Scott C. Goebel				
Duly authorized under Power of Attorney
effective as of June 1, 2008 by and on behalf of FMR LLC 
and its direct and indirect  subsidiaries


	SCHEDULE 13G - TO BE INCLUDED IN 
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)



	Pursuant to the instructions in Item 7 of Schedule 13G, 
Fidelity Management & Research Company ("Fidelity"), 82 
Devonshire Street, Boston, Massachusetts 02109, a wholly-
owned subsidiary of FMR LLC and an investment adviser 
registered under Section 203 of the Investment Advisers Act 
of 1940, is the beneficial owner of 9,792,229 shares or 
10.598% of the Common Stock outstanding of MERRIMACK 
PHARMACEUTICALS INC  ("the Company") as a result of 
acting as investment adviser to various investment companies 
registered under Section 8 of the Investment Company Act of 
1940.

	Edward C. Johnson 3d and FMR LLC, through its 
control of Fidelity, and the funds each has sole power to 
dispose of the 9,792,229 shares owned by the Funds.

	Members of the family of Edward C. Johnson 3d, 
Chairman of FMR LLC, are the predominant owners, directly 
or through trusts, of Series B voting common shares of FMR 
LLC, representing 49% of the voting power of FMR LLC.  
The Johnson family group and all other Series B shareholders 
have entered into a shareholders' voting agreement under 
which all Series B voting common shares will be voted in 
accordance with the majority vote of Series B voting common 
shares.  Accordingly, through their ownership of voting 
common shares and the execution of the shareholders' voting 
agreement, members of the Johnson family may be deemed, 
under the Investment Company Act of 1940, to form a 
controlling group with respect to FMR LLC.

	Neither FMR LLC nor Edward C. Johnson 3d, 
Chairman of FMR LLC, has the sole power to vote or direct 
the voting of the shares owned directly by the Fidelity Funds, 
which power resides with the Funds' Boards of Trustees.  
Fidelity carries out the voting of the shares under written 
guidelines established by the Funds' Boards of Trustees.

	Pyramis Global Advisors Trust Company ("PGATC"), 
900 Salem Street, Smithfield, Rhode Island, 02917, an indirect 
wholly-owned subsidiary of FMR LLC and a bank as defined 
in Section 3(a)(6) of the Securities Exchange Act of 1934, is 
the beneficial owner of 22,700 shares or 0.025% of the 
outstanding Common Stock of the MERRIMACK 
PHARMACEUTICALS INC as a result of its serving as 
investment manager of institutional accounts owning such 
shares.

	Edward C. Johnson 3d and FMR LLC, through its 
control of Pyramis Global Advisors Trust Company, each has 
sole dispositive power over 22,700 shares and sole power to 
vote or to direct the voting of 22,700 shares of Common Stock 
owned by the institutional accounts managed by PGATC as 
reported above.



	SCHEDULE 13G - TO BE INCLUDED IN 
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)
	RULE 13d-1(f)(1)  AGREEMENT

	The undersigned persons, on April 09, 2012, agree and 
consent to the joint filing on their behalf of this Schedule 13G 
in connection with their beneficial ownership of the Common 
Stock of MERRIMACK PHARMACEUTICALS INC at 
March 30, 2012.

	FMR LLC

	By /s/ Scott C. Goebel			
	Scott C. Goebel
	Duly authorized under Power of Attorney effective as 
of June 1, 2008, by and on behalf of FMR LLC and its direct 
and indirect subsidiaries

	Edward C. Johnson 3d

	By /s/ Scott C. Goebel			
	Scott C. Goebel
	Duly authorized under Power of Attorney effective as 
of June 1, 2008, by and on behalf of Edward C. Johnson 3d

	Fidelity Management & Research Company

	By /s/ Scott C. Goebel			
	Scott C. Goebel
	Senior V.P. and General Counsel