9 EXHIBIT 99.A AGREEMENT TO FILE JOINTLY We, the undersigned, hereby express our agreement that the Schedule 13D to which this Agreement is appended is filed on behalf of each of us. HOECHST MARION ROUSSEL, INC. Date: January 9, 1997 By: /s/ James P....

                                                                 9
                                  EXHIBIT 99.A

                           AGREEMENT TO FILE JOINTLY

     We, the undersigned, hereby express our agreement that the Schedule
13D to which this Agreement is appended is filed on behalf of each of us.



                                        HOECHST XXXXXX XXXXXXX, INC.



Date: January 9, 1997                   By:  /s/ Xxxxx X. Xxxxxxx
                                             Xxxxx X. Xxxxxxx
                                             Vice President and 
                                             Chief Financial Officer


                                        HMR PHARMA, INC.



Date: January 9, 1997                   By:  /s/ Xxxxxxx X. Xxxxxx
                                             Xxxxxxx X. Xxxxxx
                                             Vice President and
                                             Assistant Secretary