Antibodies to human immunodeficiency virus type 1 (HIV-1) and type 2 (HIV-2) and to human T-cell leukemia virus (HTLV-1) were investigated by ELISA, Western blot and radioimmunoprecipitation (RIPA) assay in 318 sera (191 males and 127 females) obtained from syphilitic patients. The sera from 10% of the males and 3.1% of the females were positive for HIV-1. None of the sera contained antibodies to HIV-2. Antibodies to HTLV-1-2 were present in the sera of 7.1% of the males and 4.8% of the females who were seronegative for HIV. Five out of 24 (20.8%) HIV-1 positive subjects had antibodies to HTLV-1-2 as well. Sera from another group of 58 syphilitic patients (38 males and 20 females in the Anti-Venereal Disease Department), seronegative for HIV-1 and HIV-2, who denied both i.v. drug abuse and blood transfusion, were investigated in the same manner. None of the males had antibodies to HTLV-1-2, while 2 females (10%) were positive.