7,000,000 Shares

Seattle Genetics, Inc.

Common Stock

UNDERWRITING AGREEMENT

February 4, 2004

CIBC World Markets Corp.

Banc of America Securities LLC

WR Hambrecht + Co, LLC,

as Representatives of the several

    Underwriters named in Schedule I hereto

c/o CIBC World Markets Corp.

417 5^th Avenue, 2^nd Floor

New York, New York 10016

Ladies and Gentlemen:

Seattle Genetics, Inc., a Delaware corporation (the Company) proposes, subject to the terms and conditions contained herein, to sell to you and the other underwriters named on Schedule I to this Agreement (the Underwriters), for whom you are acting as Representatives (the Representatives), an aggregate of 7,000,000 shares (the Firm Shares) of the Companys common stock, $0.001 par value per share (the Common Stock). The respective amounts of the Firm Shares to be purchased by each of the several Underwriters are set forth opposite their names on Schedule I hereto. In addition, the Company proposes to grant to the Underwriters an option to purchase up to an additional 1,050,000 shares (the Option Shares) of Common Stock from the Company. The Firm Shares and the Option Shares are collectively called the Shares.

The Company has prepared and filed in conformity with the requirements of the Securities Act of 1933, as amended (the Securities Act), and the published rules and regulations thereunder (the Rules) adopted by the Securities and Exchange Commission (the Commission) a Registration Statement (as hereinafter defined) on Form S-3 (No. 333-111269), which was declared effective on January 12, 2004, relating to the Shares and the offering thereof from time to time in accordance with Rule 415 under the Securities Act. Copies of such Registration Statement (including all amendments thereof) and of the related Preliminary Prospectus (as hereinafter defined) have heretofore been delivered by the Company to you. The term Preliminary Prospectus means any preliminary prospectus, including the documents incorporated by reference therein and any supplement or amendment to such preliminary prospectus used in connection with the offer of Shares prior to the date hereof. The term Registration Statement as used in this Agreement means the initial registration statement (including all exhibits, financial schedules and all documents and information deemed to be a

part of the Registration Statement through incorporation by reference or otherwise), as amended at the time and on the date it becomes effective (the Effective Date), including the information (if any) contained in the form of final prospectus filed with the Commission pursuant to Rule 424(b) of the Rules and deemed to be part thereof at the time of effectiveness pursuant to Rule 430A of the Rules. If the Company has filed an abbreviated registration statement to register additional Shares pursuant to Rule 462(b) under the Rules (the 462(b) Registration Statement), then any reference herein to the Registration Statement shall also be deemed to include such 462(b) Registration Statement. The term Prospectus as used in this Agreement means the prospectus in the form included in the Registration Statement at the time of effectiveness and any prospectus filed with the Commission pursuant to Rule 424(b) of the Rules. Reference made herein to any Preliminary Prospectus or to the Prospectus shall be deemed to refer to and include any documents incorporated by reference therein pursuant to Item 12 of Form S-3 under the Securities Act, as of the date of such Preliminary Prospectus or the Prospectus, as the case may be, and any reference to any amendment or supplement to any Preliminary Prospectus or the Prospectus shall be deemed to refer to and include any document filed under the Securities Exchange Act of 1934, as amended (the Exchange Act), after the date of such Preliminary Prospectus or the Prospectus, as the case may be, and incorporated by reference in such Preliminary Prospectus or the Prospectus, as the case may be.

The Company has caused to be delivered to you copies of the Preliminary Prospectus and has consented to the use of such copies for the purposes permitted by the Securities Act.

The Company understands that the Underwriters propose to make a public offering of the Shares, as set forth in and pursuant to the Prospectus, as soon after the date of this Agreement as the Representatives deem advisable. The Company hereby confirms that the Underwriters and dealers have been authorized to distribute or cause to be distributed each Preliminary Prospectus and are authorized to distribute the Prospectus (as from time to time amended or supplemented if the Company furnishes amendments or supplements thereto to the Underwriters).

1. Sale, Purchase, Delivery and Payment for the Shares. On the basis of the representations, warranties and agreements contained in, and subject to the terms and conditions of, this Agreement:

(a) The Company agrees to issue and sell to each of the Underwriters, and each of the Underwriters agrees, severally and not jointly, to purchase from the Company, at a purchase price of $7.755 per share (the Initial Price), the number of Firm Shares set forth opposite the name of such Underwriter under the column Number of Firm Shares to be Purchased from the Company on Schedule I to this Agreement, subject to adjustment in accordance with Section 6 hereof.

(b) The Company hereby grants to the several Underwriters an option to purchase, severally and not jointly, all or any part of the Option Shares at the Initial Price. The number of Option Shares to be purchased by each Underwriter shall be the same percentage (adjusted by the Representatives to eliminate fractions) of the total number of Option Shares to be purchased by the Underwriters as such Underwriter is purchasing of

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the Firm Shares. Such option may be exercised only to cover over-allotments in the sales of the Firm Shares by the Underwriters and may be exercised in whole or in part at any time on or before 12:00 noon, New York City time, on the business day before the Firm Shares Closing Date (as defined below), and from time to time thereafter within 30 days after the date of this Agreement, in each case upon written, facsimile or telegraphic notice, by the Representatives to the Company no later than 12:00 noon, New York City time, on the business day before the Firm Shares Closing Date (if exercised prior to the Firm Shares Closing Date as provided above) or at least two business days before the Option Shares Closing Date (as defined below), as the case may be, setting forth the number of Option Shares to be purchased and the time and date (if other than the Firm Shares Closing Date) of such purchase.

(c) Payment of the purchase price for, and delivery of certificates for, the Firm Shares shall be made at the offices of CIBC World Markets Corp., One World Financial Center, New York, New York 10281, at 10:00 a.m., New York City time, on the third business day following the date of this Agreement or at such time on such other date, not later than ten (10) business days after the date of this Agreement, as shall be agreed upon by the Company and the Representatives (such time and date of delivery and payment are called the Firm Shares Closing Date). In addition, in the event that any or all of the Option Shares are purchased by the Underwriters, payment of the purchase price, and delivery of the certificates, for such Option Shares shall be made at the above-mentioned offices, or at such other place as shall be agreed upon by the Representatives and the Company, on each date of delivery as specified in the notice from the Representatives to the Company (such time and date of delivery and payment are called the Option Shares Closing Date). The Firm Shares Closing Date and any Option Shares Closing Date are called, individually, a Closing Date and, together, the Closing Dates.

(d) Payment shall be made to the Company by wire transfer of immediately available funds or by certified or official bank check or checks payable in New York Clearing House (same day) funds drawn to the order of the Company, against delivery of the Shares to the Representatives for the respective accounts of the Underwriters of the Shares to be purchased by them.

(e) The Shares shall be delivered by or on behalf of the Company to the Representatives through the facilities of the Depository Trust Company (DTC) for the account of each such Underwriter unless the Underwriters direct otherwise.

2. Representations and Warranties of the Company. The Company represents and warrants to each Underwriter as of the date hereof, as of the Firm Shares Closing Date and as of each Option Shares Closing Date (if any), as follows:

(a) On the Effective Date, the Registration Statement complied, and on the date of the Prospectus, the date any post-effective amendment to the Registration Statement becomes effective, the date any supplement or amendment to the Prospectus is filed with the Commission and each Closing Date, the Registration Statement and the Prospectus (and any amendment thereof or supplement thereto) will comply, in all

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material respects, with the requirements of the Securities Act and the Rules and the Exchange Act and the rules and regulations of the Commission thereunder. The Registration Statement did not, as of the Effective Date, contain any untrue statement of a material fact or omit to state any material fact required to be stated therein or necessary in order to make the statements therein not misleading; and on the Effective Date and the other dates referred to above neither the Registration Statement nor the Prospectus, nor any amendment thereof or supplement thereto, will contain any untrue statement of a material fact or will omit to state any material fact required to be stated therein or necessary in order to make the statements therein not misleading. When any related preliminary prospectus was first filed with the Commission (whether filed as part of the Registration Statement or any amendment thereto) and when any amendment thereof or supplement thereto was first filed with the Commission, such preliminary prospectus as amended or supplemented complied in all material respects with the applicable provisions of the Securities Act and the Rules and did not contain any untrue statement of a material fact or omit to state any material fact required to be stated therein or necessary in order to make the statements therein not misleading, other than information permitted to be omitted pursuant to Rule 430A. If applicable, each Preliminary Prospectus and the Prospectus delivered to the Underwriters for use in connection with this offering was identical to the electronically transmitted copies thereof filed with the Commission pursuant to EDGAR, except to the extent permitted by Regulation S-T. Notwithstanding the foregoing, none of the representations and warranties in this paragraph 4(a) shall apply to statements in, or omissions from, the Registration Statement or the Prospectus made in reliance upon, and in conformity with, information herein or otherwise furnished in writing by the Representatives on behalf of the several Underwriters for use in the Registration Statement or the Prospectus. With respect to the preceding sentence, the Company acknowledges that the only information furnished in writing by the Representatives on behalf of the several Underwriters for use in the Registration Statement or the Prospectus in paragraphs 3, 10, 11, 12, 13, 14, 15 and 16 under the caption Underwriting in the Prospectus.

(b) The Registration Statement is effective under the Securities Act and no stop order preventing or suspending the effectiveness of the Registration Statement or suspending or preventing the use of the Prospectus has been issued by the Commission and no proceedings for that purpose have been instituted or are threatened under the Securities Act. Any required filing of the Prospectus and any supplement thereto pursuant to Rule 424(b) of the Rules has been or will be made in the manner and within the time period required by such Rule 424(b).

(c) The documents incorporated by reference in the Registration Statement and the Prospectus, at the time they became effective or were filed with the Commission, as the case may be, complied in all material respects with the requirements of the Securities Act or the Exchange Act, as applicable, and the rules and regulations of the Commission thereunder, and none of such documents contained an untrue statement of a material fact or omitted to state a material fact required to be stated therein or necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading, and any further documents so filed and incorporated by reference in the Registration Statement and the Prospectus, when such documents

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become effective or are filed with the Commission, as the case may be, will conform in all material respects to the requirements of the Securities Act or the Exchange Act, as applicable, and the rules and regulations of the Commission thereunder and will not contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein, in light of the circumstances under which they are made, not misleading; provided, however, that this representation and warranty shall not apply to any statements or omissions made in reliance upon and in conformity with information furnished in writing to the Company by an Underwriter through the Representatives expressly for use therein.

(d) The financial statements of the Company (including all notes and schedules thereto) included or incorporated by reference in the Registration Statement and the Prospectus present fairly the financial position of the Company as of the dates indicated and the results of its operations, stockholder equity and changes in their cash flows for the periods specified; said financial statements and related schedules and notes thereto, and the unaudited financial information filed with the Commission as part of the Registration Statement, have been prepared in conformity with generally accepted accounting principles, consistently applied throughout the periods involved, and the supporting schedules included in the Registration Statement present fairly the information required to be stated therein; and the pro forma financial information, and the related notes thereto, included in the Registration Statement and the Prospectus has been prepared in accordance with the applicable requirements of the Securities Act and the Exchange Act, as applicable, and is based upon good faith estimates and assumptions believed by the Company to be reasonable.

(e) There has not occurred any material adverse change, or any development involving a prospective material adverse change, in the condition, financial or otherwise, or in the earnings, business, operations or prospects of the Company, from that set forth in the Prospectus (exclusive of any amendments or supplements thereto subsequent to the date of this Agreement).

(f) PricewaterhouseCoopers LLP who have certified certain financial statements of the Company, are independent public accountants as required by the Securities Act.

(g) The Company is duly incorporated and is validly existing as a corporation in good standing under the laws of the State of Delaware, with power and authority (corporate and other) to own its properties and conduct its business as described in the Registration Statement and the Prospectus, and has been duly qualified as a foreign corporation for the transaction of business and is in good standing or, where applicable, validly existing under the laws of each other jurisdiction in which it owns or leases properties, or conducts any business, so as to require such qualification, other than where the failure to be so qualified, in good standing or validly existing would not have a material adverse effect on the general affairs, business, prospects, management, financial position, stockholders equity or results of operations of the Company, (a Material Adverse Effect).

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