EXHIBIT A AGREEMENT REGARDING JOINT FILING -------------------------------- The undersigned, Biotechnology Value Fund, L.P., a Delaware limited partnership, and BVF Partners L.P., a Delaware limited partnership, and BVF Inc., a Delaware corporation,...

 
                                   EXHIBIT A

                       AGREEMENT REGARDING JOINT FILING
                       --------------------------------


     The undersigned, Biotechnology Value Fund, L.P., a Delaware limited
partnership, and BVF Partners L.P., a Delaware limited partnership, and BVF
Inc., a Delaware corporation, hereby agree and acknowledge that the Amendment
containing the information required by Schedule 13D, to which this Agreement is
attached as an exhibit, is filed on behalf of each of them.  The undersigned
further agree that any further amendments or supplements thereto shall also be
filed on behalf of each of them.

Dated:  May 2, 1997

        BIOTECHNOLOGY VALUE FUND, L.P.                   
                                                         
        By:  BVF Partners L.P., its general partner      
                                                         
             By:  BVF Inc., its general partner          
                                                         
                  By:  /s/ XXXX X. XXXXXXX               
                       -------------------               
                       Xxxx X. Xxxxxxx                   
                       President                         
                                                         
        BVF PARTNERS L.P.                                
                                                         
        By:  BVF Inc., its general partner               
                                                         
             By:  /s/ XXXX X. XXXXXXX                    
                  -------------------                    
                  Xxxx X. Xxxxxxx                        
                  President                              
                                                         
        BVF INC.                                         
                                                         
        By:  /s/ XXXX X. XXXXXXX                         
             -------------------                         
             Xxxx X. Xxxxxxx                             
             President