EXHIBIT 2.1

                         SECOND AMENDMENT AGREEMENT
                         --------------------------

        THIS SECOND AMENDMENT AGREEMENT dated as of January 12, 1999, among 
McKESSON CORPORATION, a Delaware corporation ("McKesson"), HBO & COMPANY, a 
Delaware corporation ("HBO"), and McKESSON MERGER SUB, INC. ("Merger Sub"), a 
Delaware corporation and a wholly-owned subsidiary of McKesson.

                                 WITNESSETH:

        WHEREAS, McKesson, HBOC and Merger Sub have entered into that certain 
Agreement and Plan of Merger dated as of October 17, 1998, as amended by an 
Amendment Agreement dated as of November 9, 1998 (the "Merger Agreement"); and

        WHEREAS, the parties hereto desire to amend further the Merger 
Agreement as set forth herein;

        NOW, THEREFORE, in consideration of the covenants and agreements set 
forth herein, the parties agree as follows:

    1.  The last paragraph of Exhibit D to the Merger Agreement entitled 
                              ---------
"Surviving Corporation Directors" is hereby amended to read as follows:


                       "Surviving Corporation Directors

                  Charles W. McCall           
                  Mark A. Pulido              
                  Alfred C. Eckert, III       
                  Phillip A. Incarnati        
                  Alton F. Irby, III          
                  M. Christine Jacobs         
                  Gerald E. Mayo              
                  James V. Napier             
                  Donald C. Wegmiller"         

    2.   Except as expressly provided in this Amendment Agreement, all of the 
terms and conditions of the Merger Agreement shall remain in full force and 
effect and not be altered or amended hereby.

    3.  This Agreement may be executed in one or more counterparts, all of 
which shall be considered one and the same agreement and shall become 
effective when one or more counterparts have been signed by each of the parties
and delivered to the other parties.

    4.  This Agreement shall be governed by, and construed in accordance with,
the laws of the State of Delaware, regardless of the laws that might otherwise 
govern under applicable principles of conflict of laws thereof.

 
        IN WITNESS WHEREOF, HBO, McKesson and Merger Sub have caused this 
Amendment Agreement to be signed by their respective officers thereunto duly 
authorized, all as of the date first written above.


                                        HBO & COMPANY

                                        By:   /s/ Jay M. Lapine
                                           -------------------------------------
                                           Name:  Jay M. Lapine
                                                 -------------------------------
                                           Title: Sr. Vice President and 
                                                 -------------------------------
                                                  General Counsel and Secretary
                                                 -------------------------------


                                        McKESSON CORPORATION

                                        By: /s/ Ivan D. Meyerson
                                           -------------------------------------
                                           Name:  Ivan D. Meyerson
                                                 -------------------------------
                                           Title: Vice President
                                                 -------------------------------


                                        McKESSON MERGER SUB, INC.

                                        By: /s/ Nancy A. Miller
                                           -------------------------------------
                                           Name:  Nancy A. Miller
                                                 -------------------------------
                                           Title: Vice President & Secretary
                                                 -------------------------------

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