1. Diagnostic Reagents Laboratory, Public Health Laboratory Service, Centre for Applied Microbiology and Research, Porton Down, Salisbury, Wilts. SP4 0JG, U.K.
2. Department of Biochemistry, Imperial College, London S.W.1, U.K.
3. Blood Products Laboratory, Elstree, Herts., U.K.
4. Department of Biochemistry, University of Southampton, Southampton, U.K.