EXHIBIT 10.16

         TWELFTH ADDENDUM TO AMENDED REGISTRATION RIGHTS AGREEMENT


     This Twelfth Addendum ("Addendum") to the Amended Registration Rights
Agreement dated June 24, 1994, as amended through the date hereof ("Registration
Rights Agreement") between Ligand Pharmaceuticals Incorporated (the "Company")
and Elan International Services, Ltd. ("EIS") is effective as of August 4, 1999.

                                    RECITALS

     A. The Company has issued a warrant to purchase up to 91,406 shares of the
Company's Common Stock with an exercise price equal to $13.80 per share (the
"EIS Warrant") to EIS.

     B. This Addendum serves to include any shares of the Company's Common Stock
issuable upon the exercise of the EIS Warrant within the definition of
"Registrable Securities" under the Registration Rights Agreement and to provide
that Schedule A to the Registration Rights Agreement shall be further updated to
include any shares issued upon the exercise of the EIS Warrant, all pursuant to
Section 2.6(a) of the Registration Rights Agreement.

     NOW, THEREFORE, in consideration of the mutual promises and covenants set
forth in the Registration Rights Agreement, the parties agree as follows:

     1. Section 1.1, paragraph (f) of the Registration Rights Agreement is
hereby restated in its entirety as follows:

          "(f) The term "Registrable Securities" means (i) the Common Stock
     issuable or issued upon exercise of those warrants issued to certain
     Existing Investors and pursuant to which such Existing Investors were
     previously granted registration rights by the Company, (ii) the shares of
     Common Stock (or the shares of such other class of stock into which the
     Common Stock is converted) issuable upon conversion of those certain
     Unsecured Convertible Promissory Notes issued to American Home Products
     Corporation pursuant to the Stock and Note Purchase Agreement dated
     September 2, 1994, (iii) the 35,957 shares of Common Stock issuable or
     issued upon exercise of the Warrant issued to Genentech, Inc. in connection
     with the merger of L.G. Acquisition Corp., a wholly-owned subsidiary of the
     Company, with and into Glycomed Incorporated, which shares are reflected on
     Schedule A attached to the Fourth Addendum to this Agreement, (iv) the
     164,474 shares of Common Stock (or that number of shares of such other
     class of stock into which the Common Stock is converted) issued to S.R. One
     Limited pursuant to a Stock and Note Purchase Agreement dated February 3,
     1995 (the "Stock and Note Purchase Agreement"), which shares are reflected
     on Schedule A attached to the Eighth Addendum to this Agreement, and the
     shares of Common Stock (or the shares of such other class of stock into
     which the Common Stock is converted) issuable upon conversion of those
     certain



     Unsecured Convertible Promissory Notes dated October 30, 1997 (the
     "S.R. One Notes") issued pursuant to the Stock and Note Purchase Agreement
     (and upon such conversion of the S.R. One Notes, Schedule A shall be
     updated to include such shares), (v) the 274,423 shares of Common Stock (or
     that number of shares of such other class of stock into which the Common
     Stock is converted) issued to SmithKline Beecham plc pursuant to a Stock
     Purchase Agreement dated April 24, 1998 (the "SmithKline Stock Purchase
     Agreement"), which shares are reflected on Schedule A attached to the Ninth
     Addendum to this Agreement, and the shares of Common Stock (or the shares
     of such other class of stock into which the Common Stock is converted)
     issuable upon conversion of that certain Warrant (the "Warrant") issued
     pursuant to the SmithKline Stock Purchase Agreement (and upon such
     conversion of the Warrant, Schedule A shall be updated to include such
     shares), (vi) the 1,278,970 shares of Common Stock (or that number of
     shares of such other class of stock into which the Common Stock is
     converted) issued to Elan International Services, Ltd. pursuant to the
     Stock Purchase Agreement dated September 30, 1998, which shares are
     reflected on Schedule A attached to the Tenth Addendum to this Agreement,
     (vii) the 437,768 shares of Common Stock (or that number of shares of such
     other class of stock into which the Common Stock is converted) issued to
     Elan International Services, Ltd. pursuant to the Securities Purchase
     Agreement, dated November 6, 1998 (the "Elan Securities Purchase
     Agreement"), which shares are reflected on Schedule A attached to the
     Eleventh Addendum to this Agreement, (viii) the shares of Common Stock (or
     the shares of such other class of stock into which the Common Stock is
     converted) issuable upon conversion of the Zero Coupon Convertible Senior
     Notes due 2008 (the "Elan Notes") issued pursuant to the Elan Securities
     Purchase Agreement (and upon such conversion of the Elan Notes, Schedule A
     shall be updated to include such shares), (viii) the 429,185 shares of
     Common Stock (or the shares of such other class of stock into which the
     Common Stock is converted) issued to Elan Corporation, plc pursuant to the
     Development, Licence and Supply Agreement dated November 9, 1998 (the "Elan
     License Agreement"), which shares are reflected on Schedule A attached to
     the Eleventh Addendum to this Agreement, (ix) the shares of Common Stock
     that may be issued to Elan Corporation, plc pursuant to the Elan License
     Agreement (and upon each such issuance, Schedule A shall be updated to
     include such shares), (x) the shares of Common Stock (or the shares of such
     other class of stock into which the Common Stock is converted) issuable to
     Elan International Services, Ltd. upon exercise of that certain Warrant
     (the "EIS Warrant") dated August 4, 1999 (and upon such exercise of the EIS
     Warrant, Schedule A shall be updated to include such shares) and (xi) any
     Common Stock of the Company issued as (or issuable upon the conversion or
     exercise of any warrant, right or other security which is issued as) a
     dividend or other distribution with respect to, or in exchange for or in
     replacement of the shares referenced in (i), (ii), (iii), (iv), (v), (vi),
     (vii), (viii), (ix) and (x) above, excluding in all cases, however, any
     Registrable Securities sold by a person in a transaction in which rights
     under this Agreement are not assigned."



     2. Schedule A of the Registration Rights Agreement is hereby restated in
its entirety as attached to this Addendum.

     3. This Addendum may be executed in one or more counterparts.

     4. This Addendum shall be binding upon the Company, EIS, each holder of
Registrable Securities and each future holder of Registrable Securities pursuant
to Section 2.6(a) of the Registration Rights Agreement.


                [REMAINDER OF THIS PAGE INTENTIONALLY LEFT BLANK]




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

ELAN INTERNATIONAL SERVICES,           LIGAND PHARMACEUTICALS
LTD.                                   INCORPORATED


By:      /s/ Kevin Insley              By:       /s/ Paul Maier

Title:   President & CFO               Title:   /s/ Senior Vice President & CEO















                     [SIGNATURE PAGE TO TWELFTH ADDENDUM TO
                     AMENDED REGISTRATION RIGHTS AGREEMENT]




                                   SCHEDULE A

                                       to
                               Twelfth Addendum to
                      Amended Registration Rights Agreement



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                                                         Shares
Name                                                     Issued
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American Home Products Corporation                      374,626

American Home Products Corporation                      374,626

American Home Products Corporation                      249,749

American Home Products Corporation                      124,875

Aspen Venture Partners, L.P.                              2,659

Elan Corporation, plc                                   429,185

Elan International Services, Ltd.                     1,716,738

Enterprise Partners                                       3,745

Genentech, Inc.                                          35,957

Kleiner Perkins Caufield & Byers                          7,688

ML Venture Partners II, L.P.                              2,417

S.R. One, Limited                                       164,474

SmithKline Beecham                                      274,423

Venrock Associates                                        3,441

Venrock Associates II, L.P.                               1,540

Windsor Venture Lease Partners Ltd., Inc.                   283

         Total:                                       3,766,426
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                                      A-1