We tested serum samples from 50 hemophiliacs from Sevilla, Spain, for antibody to HTLV-III by indirect membrane immunofluorescence (IMI) and radioimmunoprecipitation with SDS polyacrylamide gel electrophoresis (RIP-SDS/PAGE). All had received commercial clotting factors from the United States with the exception of one hemophiliac who had never been transfused. Thirty-four (68%) reacted with HTLV-III-infected cells (H9/HTLV-III) by both methods, but not with the uninfected line (H9). Of 41 hemophilia-A patients tested, 28 (68%) were positive, and of nine hemophilia-B patients, six (66%) were positive. The nontransfused hemophilia-B patient was negative for antibody to HTLV-III by both methods. One patient with clinical AIDS tested positive as did six of seven with chronic unexplained lymphadenopathy. The eight individuals with AIDS or lymphadenopathy all had hemophilia A. We conclude that exposure to HTLV-III is widespread among asymptomatic hemophiliacs in Spain.