1. Department of Biochemistry, University of Birmingham, P.O. Box 363, Birmingham B15 2TT, U.K.
2. Department of Immunology, University of Birmingham, P.O. Box 363, Birmingham B15 2TT, U.K.
3. I.C.I. Pharmaceuticals, Alderley Park, Macclesfield, Cheshire SK10 4TG, U.K.