BY THE SPRING OF 1984, it was clear that AIDS was caused by at least one retrovirus, initially called human T-cell lymphotropic virus, type 111 (HTLV-III), and later called the human immunodeficiency virus (HIV).'.' Shortly after HTLV-111 (HIV) was described, antibody enzyme immunoassays were developed. After these tests were introduced into blood bank practice in March and April of 1985, a significant number of HIV enzyme immunoassay-repeatably reactive results, many later confirmed as true-positive results, were enco~ntered.~ This finding was unexpected and raised serious concern, because blood bankers had believed that most people at high risk for AIDS had voluntarily refrained from donating blood and plasma.4-' In 1985, a second strain of HIV was identified7 and designated HIV-2, with the original virus now called HIV1. A high degree (60-91%) of serologic cross-reactivity was exhibited against the original whole viral lysate enzyme immunoassays for HIV-1 antibodies8 To date, one blood donor infected with HIV-2 has been identified in the United States whose sera reacted with licensed HIV1 antibody screening tests,Y and one blood donor has been identified whose testing patterns suggested reactivity with HIV-2 antigens alone.' No case of transfusion-transmitted HIV-2 has been reported in the United States.' US Abbreviations: HIV-1 and -2 = human immunodeficiency virus type 1 and type 2; HTLV-I and -11 = human T-lymphotropic virus type I and type 11; HTLV-111 = HTLV, type 111, now HIV; MCC = medical coordinating center; NHLBI = National Heart, Lung, and Blood Institute; REDS = Retrovirus Epidemiology Donor Study; RFP(s) = request(s) for proposals; WBC(s) = white cell(s).