Blood donors were asked to participate in this multicenter study. Seropositive donors were notified of test results, counseled, and invited to join the follow-up study. Data collected during the first interview included such items as possible exposures to persons with retroviral infection, sexual preferences and contacts, marital history, intravenous drug use, occupation, travel, knowledge of AIDS and routes of HIV transmission. At subsequent visits a blood specimen was collected for laboratory testing and an assessment made of changes in behavior to reduce the risk of virus transmission.