1. Center for Infectious Diseases, Centers for Disease Control, Atlanta, Georgia 30333
2. Department of Cancer Biology, Harvard School of Public Health, Boston, Massachusetts 02115
3. American Red Cross Blood Services Laboratories, Bethesda, Maryland 20814
4. Department of Medicine, Memorial Sloan-Kettering Cancer Center, New York 10021
5. Department of Medicine, New England Deaconess Hospital, Boston, Massachusetts 02215