JOINT FILING AGREEMENT

                                   EXHIBIT F

                             JOINT FILING AGREEMENT


                  Each of the Reporting Persons hereby agrees to make this joint
filing pursuant to Rule 13d-1(f) of the Exchange Act of 1934.



Dated:  December 1, 1998

                                   THE PHARMACIA & UPJOHN COMPANY


                                   By:  /s/ Xxx  X. Xxxxxxx  
                                      ------------------------------------------
                                        Title: Secretary



                                   PHARMACIA & UPJOHN S.p.A.


                                   By:  /s/ Xxxxxx X. Xxxxxx  
                                      ------------------------------------------
                                        Title: Managing Director



                                   PHARMACIA & UPJOHN AB


                                   By:  /s/ Xxxxx X. Xxxx 
                                      ------------------------------------------
                                        Title: Executive Vice President



                                   PHARMACIA & UPJOHN INTERNATIONAL N.V.


                                   By:  /s/ Xxx Xxxxxx
                                      ------------------------------------------
                                        Title: Director



                                   PHARMACIA & UPJOHN, INC.


                                   By:  /s/ Xxx  X. Xxxxxxx  
                                      ------------------------------------------
                                        Title: Secretary