JOINT FILING AGREEMENT

   1
                                    EXHIBIT 3

                             JOINT FILING AGREEMENT


      We, the undersigned, hereby express our agreement that the attached
Schedule 13G (including all amendments thereto) is filed on behalf of each of
the undersigned.



                                     BB BIOTECH AG

Date:    July 17, 1998               By:    /s/ Xxxx Xxxxx Xxxx
                                            --------------------------
                                     Name:  Xxxx Xxxxx Xxxx
                                            --------------------------
                                            Signatory Authority

Date:   July 17, 1998                By:    /s/ Xx. Xxxxxx Xxxxxxxx
                                            --------------------------
                                     Name:  Xx. Xxxxxx Xxxxxxxx
                                            --------------------------
                                            Vice Chairman and Director


                                     BIOTECH FOCUS S.A.

Date:   July 17, 1998                By:    /s/ Xx. Xxxxxxx Xxxxxx
                                            --------------------------
                                     Name:  Xx. Xxxxxxx Xxxxxx
                                            --------------------------
                                            Signatory Authority

Date:   July 17, 1998                By:    /s/ Xx. Xxxxxx Xxxxxxx
                                            --------------------------
                                     Name:  Xx. Xxxxxx Xxxxxxx
                                            --------------------------
                                            Signatory Authority