J. KOHN, J. WHICHER+, C. WARREN* and T. O’KELLY? Department of Biochemistry, University of Surrey, Guildford, Surrey GU2 JXH and Department of Clinical Pathology, Royal Marsden Hospital, Sutton, Surrey SM2 5PT, *Department of Chemical Pathology, Bristol Royal Infirmary, Bristol BS2 8HW. *Department of Chemical Pathology, Torbay Hospital, Torquay TQ2 7AA and fSt. Luke? Hospital, Guildford, Surrey GUl 3NT, England