1. National Institute for Biological Standards and Control, Holly Hill, Hampstead, London NW3 6RB, U.K.
2. Department of Haematology, School of Medicine, University College Hospital, Chenies Mews, London WC1E 6HX, U.K.
3. Department of Biochemistry, King's College, Strand, London WC2R 2LS, U.K.