LETTER AGREEMENT

     This Letter Agreement ("Letter Agreement") is made and entered into as of
the 2nd day of February 2001, by and between ChemoCentryx, Inc., a Delaware
corporation (the "Company") and Techne Corporation, a Minnesota corporation
("Techne").

                                RECITALS

          WHEREAS, the Company and Techne previously entered into an Investment
Agreement dated November 18, 1997, pursuant to which Techne purchased shares of
the Company's Series A Preferred Stock (the "Series A Stock") and was granted
warrants to purchase shares of the Series A Stock (the "Warrants"); and

          WHEREAS, the Company and Techne desire to amend the terms of the
Warrants.

                                AGREEMENT

          NOW, THEREFORE, the parties hereby agree as follows:

          1.  Relation to the Warrants.  Except as hereby amended, the Warrants
shall continue in full force and effect.

          2.  Amendment.  The Warrants are hereby amended so that they shall
not expire until December 31, 2005.

          3.  Miscellaneous.  This Letter Agreement and all acts and
transactions pursuant hereto and the rights and obligation of the parties
hereto shall be governed, construed and interpreted in all respects by the laws
of the State of California, without regard to the conflict of law provisions
thereof.

          4.  Counterparts.  This Letter Agreement may be executed in
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

                         [Signature Page Follows]

     The parties have executed this Letter Agreement as of the date first above
written.

                                                      COMPANY:

                                                      CHEMOCENTRYX, INC.
                                                      By:
                                                      Name:
                                                      Title:


                                                      TECHNE:

                                                      TECHNE CORPORATION
                                                      By:
                                                      Name:
                                                      Title: