FIRST SUPPLEMENTAL INDENTURE

EXHIBIT 4.1

FIRST SUPPLEMENTAL INDENTURE

FIRST SUPPLEMENTAL INDENTURE (this “First Supplemental Indenture”) dated as of July 12, 2005 by and among Corixa Corporation, a Delaware corporation (the “Company”), and Wells Fargo Bank, National Association as trustee (the “Trustee”).

WHEREAS, the Company has executed and delivered to the Trustee an Indenture (the “Indenture”), dated as of June 13, 2003, providing for the issuance from time to time of the Securities;

WHEREAS, the Company proposes in and by this First Supplemental Indenture to supplement and amend the Indenture in certain respects as it applies to the Company and the Securities;

WHEREAS, pursuant to the Agreement and Plan of Merger, dated as of April 29, 2005, by and among the Company, SmithKline Beecham Corporation, a Pennsylvania corporation (“GSK”), and GSK Delaware Corp., the Company will become a wholly owned subsidiary of GSK effective on or about July 12, 2005 (the “Merger”) and each outstanding share of the Common Stock will be cancelled in exchange for a right to receive $4.40 in cash at the effective time of the Merger;

WHEREAS, the Merger complies with the provisions of Section 7.1 of the Indenture;

WHEREAS, the Company, as a wholly owned subsidiary of GSK following the Merger, desires to continue to be bound by the covenants and obligations of the Company as set forth in the Indenture and the Securities on the terms and conditions set forth herein;

WHEREAS, Section 12.11 of the Indenture requires the Company, as the surviving corporation in the Merger, to execute with the Trustee a supplemental indenture providing that each Security shall be convertible into the amount of cash receivable upon consummation of the Merger by a Holder of the number of shares of Common Stock issuable upon conversion of such Security immediately prior to the consummation of the Merger; and

WHEREAS, the Company has requested that the Trustee execute and deliver this First Supplemental Indenture.

NOW, THEREFORE, the Company and the Trustee hereby agree for the equal and ratable benefit of the Holders of the Securities:

Section 1.         Definitions. Capitalized terms used herein and not defined herein have the meanings ascribed to such terms in the Indenture.

Section 2.             Conversion of the Securities. Pursuant to Section 12.11 of the Indenture, following and subject to the effectiveness of the Merger, the Securities shall be convertible, during the period specified in Section 12.1, only into the product of (x) $4.40, multiplied by (y) the number of shares (including fractional shares) of Common Stock issuable upon conversion of such Securities immediately prior to the Merger.