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                                                                    EXHIBIT 10.8

                              ZYMOGENETICS, INC.

                          SERIES B CO-SALE AGREEMENT
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                           SERIES B CO-SALE AGREEMENT

     This SERIES B CO-SALE AGREEMENT ("Agreement") is entered into as of the
_________________________ by and among the persons identified on Schedule A
hereto (collectively, the "Holders", and individually, a "Holder") and
ZymoGenetics, Inc., a Washington corporation (the "Company").

                              W I T N E S S E T H

     WHEREAS, the Holders are parties to the Series B Preferred Stock Purchase
Agreement dated October 20, 2000, pursuant to which the Purchasers are
purchasing shares of the Company's Series B Convertible Preferred Stock ("Series
B Stock") and desire to protect their individual and collective interest in said
stock.

     NOW, THEREFORE, in consideration of the premises and the mutual promises
set forth in this Agreement;

                         THE PARTIES AGREE AS FOLLOWS:

1.   Definitions

     (a)  The term "Holder" includes any assignee of a Holder.

     (b)  The term "Selling Holder" means any Holder who wishes to sell Series B
Stock (or Common Stock issued upon conversion thereof) to a Strategic Investor
(as defined below).

     (c)  The term "Shares" means shares of Series B Stock or Common Stock
issued upon conversion thereof, together with any other shares of capital stock
of the Company issued as a dividend thereon or in connection with a stock split
or other recapitalization thereof.

     (d)  The term "Strategic Investor" means any company whose primary
business, or a substantial portion of whose business, directly or indirectly,
consists of the research, development, manufacture, marketing, licensing,
distribution and/or sale of pharmaceutical and/or biotechnology products.

     (e)  The term "Transfer" means any transfer, sale, assignment or other
disposition by a Holder of Series B Stock to a Strategic Investor (other than a
Permitted Transferee (as defined below)).

     (f)  The term "Permitted Transferee" means, with respect to any Holder, (i)
any trustee, nominee, or custodian of such Holder; (ii) any unit holder,
shareholder,
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partner, participant, manager, or adviser (or an employee of such manager or
adviser) in any such Holder; or (iii) any investment fund (or its trustee,
nominee or custodian) managed or advised by the same manager or adviser as such
Holder.

2.   Co- Sale Provisions

     2.1  Notice of Transfer

     If at any time a Selling Holder proposes a Transfer, then the Selling
Holder shall give each Holder and the Company written notice of the Selling
Holder's intention to effect the Transfer (the "Transfer Notice"), which
Transfer Notice shall include (i) a description of the number and type of Shares
to be transferred ("Offered Shares"), (ii) the total number of Shares owned by
the Selling Holder, (iii) the identity and address of the Strategic Investor who
is the prospective transferee ("Transferee") and (iv) the consideration and the
material terms and conditions upon which the proposed Transfer is to be made.
The Transfer Notice shall certify that the Selling Holder has received a firm
offer from the Transferee and in good faith believes a binding agreement for the
Transfer is obtainable on the terms set forth in the Transfer Notice. The
Transfer Notice shall also include a copy of any written proposal, term sheet or
letter of intent or other agreement relating to the proposed Transfer.

     2.2  Participation Right

     Each Holder shall have the right to request, as a condition to any Transfer
by the Selling Holder and exercisable upon written notice in accordance with
Section 2.4 below, that the Transferee purchase from such Holder up to that
portion (determined in accordance with Section 2.3 below) of such Holder's
Shares at the same price per share and on the same terms and conditions as those
offered to the Selling Holder.

     2.3  Determination of Number of Shares Included

     For purposes of this Section 2, the maximum number of Shares that any
Holder may request that the Transferee purchase in connection with the Transfer
shall be that number of Shares as is determined by multiplying the total number
of Offered Shares by a fraction, the numerator of which is the number of Shares
owned by such Holder and the denominator of which is the total number of Shares
then outstanding.

     2.4  Notice of Participation

     Each Holder electing to exercise its rights under this Section 2 to
participate in a Transfer (a "Participant") shall notify the Selling Holder and
the Transferee in writing of such election within twenty (20) days after
receiving the Transfer Notice,

                                      -2-
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which notice shall include the number of Shares (up to the number determined
pursuant to Section 2.3 above).

     2.5  Closing

     Unless the Transferee elects not to proceed with the purchase and sale of
the Offered Shares, a closing with respect to such purchase and sale shall occur
at such place and at such time as the Transferee and the Selling Holder shall
mutually agree (the "Closing"). Notice of the place and time of the Closing,
together with a copy of the definitive agreement or agreements ("Definitive
Agreement") between the Transferee, on the one hand, and the Selling Holder and
each Participant, on the other hand, pursuant to which the Transfer shall be
effected (which Definitive Agreement shall be mutually acceptable to the
Transferee and the Selling Holder and include the terms and conditions set forth
in the Transfer Notice), shall be provided to each Participant at least five (5)
business days before the date thereof.

     2.6  Mechanics of Participation

     Prior to the Closing, as a condition to each Participant's right to
participate in the Transfer, such Participant shall (i) enter into the
Definitive Agreement (upon terms and conditions no less favorable to such
Participant than to the Selling Holder), and (ii) deliver to the Selling Holder
for transfer to the Transferee one or more certificates, properly endorsed for
transfer, which represent the type and number of shares that such Participant
elects to sell to the Transferee in connection with the Transfer; provided,
                                                                  --------
however, that if the Transferee objects to the delivery of Preferred Stock in
-------
lieu of Common Stock, such Participant shall convert such Preferred Stock into
Common Stock and deliver Common Stock. The Company agrees to make any such
conversion concurrent with the actual transfer of such Shares to the Transferee.

     2.7  Payment of Purchase Price

     The stock certificate or certificates that a Participant delivers to a
Selling Holder pursuant to Section 2.6 shall be transferred to the Transferee
pursuant to the terms and conditions set forth in the Definitive Agreement, and
the Selling Holder or the Transferee shall concurrently remit to such
Participant that portion of the Transfer proceeds to which such Participant is
entitled by reason of its participation in such Transfer.

     2.8  Prohibited Transfers

     No Holder shall Transfer any of its Shares other than in compliance with
this Section 2. Any purported or attempted Transfer in violation of this Section
2 shall be null and void, and the Company shall not give effect thereto.

                                      -3-
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3.   Notices

     Any notice required or permitted by any provision of this Agreement shall
be given in writing and shall be delivered personally, by nationally recognized
overnight courier, by facsimile, or by registered or certified mail, postage
prepaid, addressed (i) in the case of a Holder, to the Holder's address as set
forth in Schedule A hereto or such other address as the Holder may designate in
writing from time to time, (ii) in the case of the Company, to its principal
office. Notices that are mailed shall be deemed effective five (5) days after
deposit in the United States mail. Notices sent by nationally recognized
overnight courier shall be deemed effective one (1) business day after they have
been so sent. Notices sent by facsimile shall be deemed effective upon confirmed
facsimile transmission. Notices sent by personal delivery shall be deemed
effective upon actual receipt.

4.   Further Instruments and Actions

     The parties agree to execute such further instruments and to take such
further action as may reasonably be necessary to carry out the intent of this
Agreement. The Selling Holder agrees to cooperate affirmatively with the Company
and the Holders, to the extent reasonably requested by the Company or the
Holders, to enforce rights and obligations pursuant hereto.

5.   Term

     This Agreement shall terminate upon the earliest of (i) the closing of a
Qualified Public offering, as that term is defined in Section 2.2.2(a) of
Article II of the Company's Amended and Restated Articles of Incorporation (or
successor provision), (ii) the closing of the Company's sale of all or
substantially all of its assets or the acquisition of the Company by another
entity by means of merger, consolidation or other transaction or series of
related transactions resulting in the exchange of the outstanding shares of the
Company's capital stock such that the shareholders of the Company prior to such
transaction own, directly or indirectly, less than 50% of the voting power of
the surviving entity and (iii) four (4) years from the date of this Agreement.

6.   Entire Agreement

     This Agreement contains the entire understanding of the parties hereto with
respect to the subject matter hereof, supersedes all other agreements between or
among any of the parties with respect to the subject matter hereof and cannot be
altered or otherwise amended except pursuant to an instrument in writing signed
by each of the parties to this Agreement. This Agreement shall be interpreted
under the laws of the State of Washington without reference to conflicts of law
provisions.

                                      -4-
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7.   Amendments and Waivers; Governing Law; Jurisdiction; Venue

     This Agreement may be terminated, any term of this Agreement may be amended
and the observance of any term of this Agreement may be waived (either generally
or in a particular instance and either retroactively or prospectively), only
with the written consent of the Company and the holders of more than 50% of the
then outstanding Series B Stock (or Common Stock issued upon conversion
thereof). Any amendment or waiver effected in accordance with this paragraph
shall be binding upon all Holders and their respective successors and assigns.
The parties irrevocably consent to the jurisdiction and venue of the state and
federal courts located in King County Washington in connection with any action
relating to this Agreement.

8.   Severability

     In case any provision of the Agreement shall be invalid, illegal or
unenforceable, the validity, legality and enforceability of the remaining
provisions shall not in any way be affected or impaired thereby.

9.   Professional Fees

     In the event that any dispute among the parties to this Agreement should
result in litigation, the prevailing party in such dispute shall be entitled to
recover from the losing party all fees, costs and expenses of enforcing any
right of such prevailing party under or with respect to this Agreement,
including without limitation, such reasonable fees and expenses of attorneys and
accountants, which shall include, without limitation, all fees, costs and
expenses of appeals.

10.  Counterparts

     This Agreement may be executed in two or more counterparts, each of which
shall be deemed an original, but all of which together shall constitute one and
the same instrument.

11.   Condition to Transfer

      No Holder may transfer, assign or otherwise dispose of its Shares unless
the person receiving said Shares has consented, in writing, to be bound by the
terms and conditions of this Agreement. Any purported or attempted Transfer in
violation of this Section 11 shall be null and void, and the Company shall not
give effect thereto.

                                      -5-
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12.  Legends

     Certificates representing any Series B Stock, and any Common Stock issued
upon conversion thereof, shall be imprinted with a legend in the following form:

     THE SECURITIES REPRESENTED BY THIS CERTIFICATE MAY NOT BE
     SOLD, ASSIGNED OR OTHERWISE TRANSFERRED EXCEPT IN COMPLIANCE
     WITH A SERIES B CO-SALE AGREEMENT AMONG ZYMOGENETICS, INC.
     AND CERTAIN HOLDERS OF THE COMPANY'S SERIES B CONVERTIBLE
     PREFERRED STOCK. A COPY OF THIS AGREEMENT IS AVAILABLE FROM
     THE SECRETARY OF THE COMPANY.

     Upon the request of a Holder following termination of this Agreement
pursuant to Section 5, the Company shall promptly prepare stock certificates
which do not contain such legend and shall substitute such new stock
certificates for the certificates then held by such Holder upon surrender of
such legended certificates.

                                      -6-
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     IN WITNESS WHEREOF, the parties have executed this Agreement as of the date
first above written.

                                       ZYMOGENETICS, INC.



                                       By: /s/Bruce L.A. Carter
                                           --------------------------------
                                           Bruce L.A. Carter
                                           Its President and CEO


                                       HOLDERS:



                                       WARBURG, PINCUS EQUITY
                                       PARTNERS, L.P.

                                       By: Warburg, Pincus & Co.
                                           Its General Partner



                                           By: /s/Jonathan Leff
                                               -----------------------------
                                               Jonathan Leff, Partner

                                      -7-
<PAGE>

                                           WARBURG, PINCUS NETHERLANDS
                                           EQUITY PARTNERS I, C.V.

                                           By: Warburg, Pincus & Co.
                                               Its General Partner



                                               By: /s/ Jonathan Leff
                                                   ---------------------------
                                                   Jonathan Leff, Partner


                                           WARBURG, PINCUS NETHERLANDS
                                           EQUITY PARTNERS II, C.V.

                                           By: Warburg, Pincus & Co.
                                               Its General Partner


                                               By: /s/ Jonathan Leff
                                                   ---------------------------
                                                   Jonathan Leff, Partner


                                           WARBURG, PINCUS NETHERLANDS
                                           EQUITY PARTNERS III, C.V.

                                           By: Warburg, Pincus & Co.
                                               Its General Partner


                                               By: /s/ Jonathan Leff
                                                   ---------------------------
                                                   Jonathan Leff, Partner

                                      -8-
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                                         APAX EXCELSIOR VI, L.P.

                                         By: Apax Excelsior VI Partners, L.P.
                                             Its General Partner

                                         By: Patricof & Co. Managers, Inc.
                                             its General Partner

                                         By: /s/ Lori Rafield
                                             -----------------------------------
                                         Name:  Lori Rafield, PhD
                                         Title: Vice President


                                         APAX EUROPE IV - A, L.P.
                                         (Delaware USA Limited Partnership)

                                         /s/ C.A. Helyar
                                         ---------------------------------------
                                         C.A. Helyar, Director

                                         For and on behalf of Apax Europe IV GP
                                         Co. Limited acting in its capacity as
                                         managing general partner of Apax Europe
                                         IV GP, L.P. acting in its capacity as
                                         managing general partner of Apax Europe
                                         IV - A, L.P.


                                         APAX EUROPE IV - B, L.P.
                                         (English Limited Partnership)

                                         /s/ C.A. Helyar
                                         ---------------------------------------
                                         C.A. Helyar, Director

                                         For and on behalf of Apax Europe IV GP
                                         Co. Limited acting in its capacity as
                                         managing general partner of Apax Europe
                                         IV GP, L.P. acting in its capacity as
                                         managing general partner of Apax Europe
                                         IV - B, L.P.

                                      -9-
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                                         APAX EUROPE IV C, GMBH & CO.KG
                                         (German Limited Partnership)

                                         /s/ C.A. Helyar
                                         ---------------------------------------
                                         C.A. Helyar, Director

                                         For and on behalf of Apax Europe IV GP
                                         Co. Limited acting in its capacity as
                                         managing general partner of Apax Europe
                                         IV GP, L.P. acting in its capacity as
                                         managing general partner of Apax Europe
                                         IV C Gmbh & Co.KG


                                         APAX EUROPE IV - D, L.P.
                                         (English Limited Partnership)

                                         /s/ C.A. Helyar
                                         ---------------------------------------
                                         C.A. Helyar, Director

                                         For and on behalf of Apax Europe IV GP
                                         Co. Limited acting in its capacity as
                                         managing general partner of Apax Europe
                                         IV GP, L.P. acting in its capacity as
                                         managing general partner of Apax Europe
                                         IV - D, L.P.


                                         APAX EUROPE IV - E, L.P.
                                         (English Limited Partnership)

                                         /s/ C.A. Helyar
                                         ---------------------------------------
                                         C.A. Helyar, Director

                                         For and on behalf of Apax Europe IV GP
                                         Co. Limited acting in its capacity as
                                         managing general partner of Apax Europe
                                         IV GP, L.P. acting in its capacity as
                                         managing general partner of Apax Europe
                                         IV - E, L.P.

                                      -10-
<PAGE>

                                         APAX EUROPE IV - F, C.V.
                                         (Dutch Limited Partnership)

                                         /s/ C.A. Helyar
                                         ---------------------------------------
                                         C.A. Helyar, Director

                                         For and on behalf of Apax Europe IV GP
                                         Co. Limited acting in its capacity as
                                         managing general partner of Apax Europe
                                         IV GP, L.P. acting in its capacity
                                         managing general partner of Apax Europe
                                         IV - F, C.V.


                                         APAX EUROPE IV - G, C.V.
                                         (Dutch Limited Partnership)

                                         /s/ C.A. Helyar
                                         ---------------------------------------
                                         C.A. Helyar, Director

                                         For and on behalf of Apax Europe IV GP
                                         Co. Limited acting in its capacity as
                                         managing general partner of Apax Europe
                                         IV GP, L.P. acting in its capacity as
                                         managing general partner of Apax Europe
                                         IV - G, C.V.


                                         APAX EUROPE IV - H, GmbH & CO., K.G.
                                         (German Limited Partnership)

                                         /s/ C.A. Helyar
                                         ---------------------------------------
                                         C.A. Helyar, Director

                                         For and on behalf of Apax Europe IV GP
                                         Co. Limited acting in its capacity as
                                         managing general partner of Apax Europe
                                         IV GP, L.P. acting in its capacity as
                                         attorney of Apax Europe IV - H,

                                      -11-
<PAGE>

                                         GmbH & Co.K.G.


                                         FRAZIER HEALTHCARE III, L.P.

                                         By: FHM III, L.L.C.
                                             Its General Partner

                                         By: /s/Nader Naini
                                             -----------------------------------
                                         Name:  Nader Naini
                                         Title: Managing Member

                                         FRAZIER AFFILIATES III, L.P.

                                         By: FHM III, L.L.C.
                                             Its General Partner

                                         By: /s/Nader Naini
                                             ----------------------------------
                                         Name:  Nader Naini
                                         Title: Managing Member


                                         PATRICOF PRIVATE INVESTMENT CLUB
                                          III, L.P.

                                         By: Apax Excelsior VI Partners, L.P.
                                             Its General Partner

                                         By: Patricof & Co. Managers, Inc.
                                             Its General Partner

                                         By: /s/Lori Rafield
                                             ----------------------------------
                                         Name:  Lori Rafield, PhD
                                         Title: Vice President


                                      -12-
<PAGE>

                                           VERTICAL FUND ASSOCIATES, L.P.
                                    By:    Vertical Group, L.P.
                                           Its General Partner

                                    By: /s/Stephen D. Baksa
                                        ----------------------------------------
                                    Name:  Stephen D. Baksa
                                    Title: General Partner



                                    NOVO A/S


                                    By: /s/Kurt A. Nielsen
                                        ----------------------------------------
                                    Name:   Kurt Anker Nielsen
                                    Title:  Deputy CEO and Corporate Executive
                                            Vice President



                                    SILVER TIDE HOLDING S.A.

                                    By: /s/Ernesto Bertarelli
                                        ----------------------------------------
                                    Name: Ernesto Bertarelli


                                    /s/George Rathmann
                                    --------------------------------------------
                                    Dr. George Rathmann


                                    /s/David Hirsh
                                    --------------------------------------------
                                    Dr. David Hirsh


                                    /s/Daniel Rifkin
                                    --------------------------------------------
                                    Dr. Daniel Rifkin


                                    /s/Edward Skolnik
                                    --------------------------------------------
                                    Dr. Edward Skolnik

                                      -13-