Exhibit 32.1

CERTIFICATION PURSUANT TO 18 U.S.C. SECTION 1350,

AS ADOPTED PURSUANT TO SECTION 906

OF THE SARBANES-OXLEY ACT OF 2002

In connection with the quarterly report of ZymoGenetics, Inc. (the “Company”) on Form 10-Q for the period ended March 31, 2010 as filed with the Securities and Exchange Commission on the date hereof (the “Form 10-Q”), I, Douglas E. Williams, Chief Executive Officer and James A. Johnson, Chief Financial Officer, of the Company, certify, pursuant to 18 U.S.C. Section 1350, as adopted pursuant to §906 of the Sarbanes-Oxley Act of 2002, that, to my knowledge:

(1) The Form 10-Q fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934; and

(2) The information contained in the Form 10-Q fairly presents, in all material respects, the financial condition and results of operations of the Company.

Dated: May 4, 2010