Sera of 65 hemophiliacs, 85 hemodialysis patients and 304 healthy volunteer blood donors in Japan were tested for antibodies against both HTLV-I and -III by an indirect immunofluorescence and radioimmunoprecipitation assay. The results showed that 10 (15.4%) of the hemophiliacs were anti-HTLV-III positive, while all other subjects were anti-HTLV-III negative. All groups contained subjects positive for anti-HTLV-I. The prevalence of the antibody in the hemophiliacs (17.4%) and hemodialysis patients (9.4%) was significantly high as compared to that of healthy controls (1%).