There is compelling evidence that the acquired immunodeficiency syndrome (AIDS) is caused by a retrovirus known as human T-cell lymphotropic virus type III (HTLV-III) or lymphadenopathy-associated virus.1,2 An enzyme-linked immunosorbent assay (ELISA) is available to detect antibodies to HTLV-III. Epidemiologic studies using the ELISA test have shown an alarming increase in the prevalence of HTLV-III antibodies among certain highrisk groups. The prevalence of seropositive homosexual men tested in a clinic for the treatment of sexually transmitted disease in San Francisco, for example, has risen from 1% in 1978 to 65% in 1984.3 Persons having hemophilia A are also at risk for AIDS, because factor VIII concentrate is manufactured from pooled plasma. In a study of a California clinic, more than 85% of patients with hemophilia A had antibodies to HTLV-III, presumably as a result of exposure to contaminated blood products.4 The full meaning of HTLV-III seropositivity is