1. Division of Parasitology, National Institute for Medical Research, Mill Hill, London NW7 1AA, U.K.
2. State Serum Institute, DK-2300 Copenhagen S, Denmark
3. The Scripps Research Institute, La Jolla, Caolifornia 92037, U.S.A.
4. Department of Pathology, University of Cambridge, Cambridge CB2 1QP, U.K.