EXHIBIT 10.6


                                    AMENDMENT
                                       TO
              COMMON STOCK AND CONVERTIBLE NOTE PURCHASE AGREEMENT

                                 April 14, 1999


                  This Amendment is made and entered into as of the date first
written above by and between Genentech, Inc., a Delaware corporation having its
principal executive office at 1 DNA Way, South San Francisco, California 94080
("Genentech"), and XOMA Ltd., a Bermuda company having its principal office at
2910 Seventh Street, Berkeley, California 94710 ("XOMA"), to amend that certain
Common Stock and Convertible Note Purchase Agreement, dated as of April 22, 1996
(the "Purchase Agreement"), by and between Genentech and XOMA.

                  Genentech and XOMA agree to amend the Purchase Agreement as
follows:

                  1.     To replace the words "State of Delaware" in Section
4(a) with the words "Commonwealth of Bermuda."

                  2.     To insert the words "and each amendment thereto" after
the words "this Agreement, the Collaboration Agreement and the Convertible Note"
in the first and second sentences of Section 4(b).

                  3.     To delete the words "federal and state" from the first
and second sentences of Section 4(c) and to replace them with the words "Bermuda
Companies Act and Bermuda", and to delete the words "after giving effect to the
filing of the Certificate of Designation with the Secretary of State of the
State of Delaware as contemplated by Section 8" from the third sentence of
Section 4(c).

                  4.     To insert the words "and each amendment thereto" after
the words "this Agreement, the Collaboration Agreement and the Convertible Note"
in Section 4(d), and to delete clause (i) from Section 4(d) and renumber clauses
(ii), (iii) and (iv) as (i), (ii) and (iii).

                  5.     To delete the words "the SEC Reports (as defined in
Section 4(j) below)" in Section 4(e) and replace them with the words "the
Company's most recent report filed with the Securities and Exchange Commission
pursuant to the Exchange Act (as defined below) that calls for such disclosure".

                  6.     In Section 4(h), (i) to delete the words "SEC Reports
and except for the issuance of the Company's Non-Voting Cumulative Convertible
Preferred Stock, Series D," and to replace them with the words "the Company's
most recent report filed with the Securities and Exchange Commission pursuant to
the Exchange Act (as defined below) that calls for such disclosure"; (ii) to
delete the words "the SEC Reports" and replace them with the words "such
report"; and (iii) to delete the word "hereof" and replace it with the word
"thereof."

                  7.     To delete the words "Amended and Restated Certificate
of Incorporation or Bylaws" in Section 4(i) and replace them with the words
"Memorandum of Continuance or Bye-Laws", and to insert at the end of Section
4(i) the words "together with each amendment thereto".

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                                      -2-

                  8.     To add the following sentence to the end of Section
6(d):

                         "Any stop-transfer instructions with respect to the
                         Securities shall be removed by the Company promptly
                         upon the request of the Holder if the Holder shall have
                         obtained an opinion of counsel in form and in substance
                         satisfactory to the Company to the effect that such
                         instructions are no longer required."

                  9.     To delete the words "upon any such assignment, sale or
transfer of such shares" from the last sentence of Section 7(a) and replace them
with the words "as set forth therein."

                  10.    To add the following as new Sections 8(c) and 8(d):

                         "(c) The Company shall use its commercially reasonable
                         efforts to remain qualified to use Form S-3 under the
                         Securities Act or another appropriate form (including
                         but not limited to Form A or Form B proposed by the
                         staff of the Securities and Exchange Commission in
                         Securities Act Release No. 33-7606, if adopted)
                         permitting registration of the Conversion Shares and
                         any Registrable Securities issued in respect of such
                         Conversion Shares for resale by the Holder in the
                         manner or manners reasonably designated by the Holder.

                         (d) The Company shall not issue any Series B Preference
                         Shares to any other party without the prior written
                         consent of the Holder."

                  11.    To insert the words "as amended" into Section 9(a)(i)
after the words "Convertible Note."

                  12.    To insert the words "and each amendment thereto" after
the words the "Collaboration Agreement and the Convertible Note" in Section
9(a)(v) and after the words "this Agreement, the Collaboration Agreement and the
Convertible Note" in Section 9(a)(vi).

                  13.    To insert the words "and each subsequent loan as of
their respective Closing Dates" into Section 9(a)(viii) after the words "Initial
Loan."

                  14.    To add a new Section 10(b)(iii) that reads in full as
follows:

                              "(iii) The Company agrees to file with the
                         Commission,promptly following the execution of the
                         Amendment to this Agreement, but in no event later than
                         45 days thereafter, a Shelf Registration Statement
                         covering not less than that number of Conversion Shares
                         that would be issued upon the conversion of the
                         Preferred Stock that would be issued if $2 million of
                         the principal amount of the Convertible Note were to be
                         converted into Preferred Stock as of the filing date.
                         Such Shelf Registration Statement shall be on Form S-3
                         under the Securities Act or another appropriate form
                         (including but not limited to Form A or Form B proposed
                         by the staff of the Securities and Exchange Commission
                         in Securities Act Release No. 33-7606, if adopted)
                         permitting registration of such Conversion Shares and
                         any Regis-


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                                      -3-


                         trable Securities issued in respect of such Conversion
                         Shares for resale by the Holder in the manner or
                         manners reasonably designated by the Holder. The
                         Company shall use its commercially reasonable efforts
                         to cause such Shelf Registration Statement to be
                         declared effective pursuant to the Securities Act as
                         promptly as practicable following the filing thereof
                         and to keep continuously effective under the Securities
                         Act until termination of such obligation pursuant to
                         Section 10(f)(iii).

                  15.    To add a new Section 10(b)(iv) that reads in full as
follows:

                              "(iv) At such time (if any) that, as a result of
                         the Purchaser's sales of Conversion Shares covered by
                         the Shelf Registration Statement referred to in Section
                         10(b)(iii), such Shelf Registration Statement is
                         available to cover sales of a maximum number of
                         Conversion Shares then issued or issuable upon
                         conversion of Preferred Stock issued or issuable upon
                         conversion of $1 million of the principal amount of the
                         Convertible Note or less, then the Company shall
                         promptly file an amendment or supplement to such
                         Registration Statement, such that such Registration
                         Statement will be available for sales of Conversion
                         Shares then issued or issuable upon conversion of
                         Preferred Stock issued or issuable upon conversion of
                         not less than $2 mi1lion of the principal amount of the
                         Convertible Note. The Company shall use its
                         commercially reasonable efforts to cause such
                         amendments or supplements to such Shelf Registration
                         Statement to be declared effective pursuant to the
                         Securities Act as promptly as practicable following the
                         filing thereof and to keep the same continuously
                         effective under the Securities Act until termination of
                         such obligation pursuant to Section 10(f)(iii)."

                  To add a new Section 10(f)(iii) that reads in full as follows:

                                "(iii) The registration obligations of the
                           Company pursuant to Sections 10(b) through (d) of
                           this Agreement shall terminate with respect to the
                           Shelf Registration Statement contemplated by Section
                           10(b)(iii) and any amendments or supplements thereto
                           contemplated by Section 10(b)(iv) at the time at
                           which (A) the Purchaser has no obligation to make
                           additional loans to the Company under the Convertible
                           Note, and (B) all of the Conversion Shares that could
                           be issued upon the conversion of the Preferred Stock
                           that could be issued if the entire principal amount
                           of the Initial Loan and each Tranche may be sold
                           within a given three-month period without compliance
                           with the registration requirements of the Securities
                           Act pursuant to Rule 144 or other applicable
                           exemption supported by a written opinion of legal
                           counsel for the Company which shall be reasonably
                           satisfactory in form and substance to legal counsel
                           for the Holder."

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                                      -4-

                  17.    To replace the notice provisions in Section 11(a) and
(b) in their entirety with the following:

                           "(a)     If to the Purchaser, to:

                                    Genentech, Inc.
                                    1 DNA Way
                                    South San Francisco, CA  94080-4990
                                    Telephone:  (650) 225-1000
                                    Facsimile:  (650) 952-9881

                                    Attention:  Corporate Secretary

                           (b)      If to the Company, to:

                                    XOMA Ltd
                                    2910 Seventh Street
                                    Berkeley, CA  94710
                                    Telephone:  (510) 664-1170
                                    Facsimile:  (510) 649-7571

                                    Attention:  Corporate Secretary"

                  18.    To replace all references to "XOMA Corporation" with
references to "XOMA Ltd.", which is a Bermuda company that is the successor in
interest to XOMA Corporation, a Delaware company.

                  19.    To replace all references to "Series E Preferred Stock"
and "Preferred Stock" with references to "Series B Preference Shares."

                  20.    To replace all references to "Common Stock" with
references to "Common Shares."

                  All other terms and conditions of the Agreement shall remain
unchanged by this Amendment. The parties have agreed that this Amendment will be
governed by and construed in accordance with the laws of the State of Delaware.

                  This Purchase Agreement may be executed in two counterparts,
each of which will be deemed an original, but both of which together will
constitute one and the same instrument.

                            [signature page follows]


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                  IN WITNESS WHEREOF, this Purchase Agreement has been executed
and delivered on the date first written above by duly authorized representatives
of the parties hereto.


GENENTECH, INC.                              XOMA LTD.
By: /s/ Louis J. Lavigne, Jr                 By: /s/ Christopher J. Margolin
    -----------------------------------      -------------------------------
    Name:   Louis J. Lavigne, Jr.                Name:   Christopher J. Margolin
    Title:  Executive Vice President and         Title:  Vice President, 
            Chief Financial Officer                      General Counsel and 
                                                         Secretary