EXHIBIT 99.1 Certification Accompanying Periodic Report Pursuant to Section 906 of the Sarbanes-Oxley Act of 2002 (18 U.S.C.ss.1350) Each of the undersigned officers of XOMA Ltd. (the "Company") hereby certify that (1) the Annual Report on Form 10-K for the year ended December 31, 2002 (the "Report") 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 Report fairly presents, in all material respects, the financial condition and results of operations of the Company. XOMA Ltd. Dated: March 14, 2003 By: /s/ John L. Castello ------------------------------------ John L. Castello Chairman of the Board, President and Chief Executive Officer Dated: March 14, 2003 By: /s/ Peter B. Davis ------------------------------------ Peter B. Davis Vice President, Finance and Chief Financial Officer