Synthetic peptides from the major envelope protein of HTLV-I (ENV-I, amino acid 177-213) and HTLV-II (ENV-II, amino acid 173-209) and a conserved region of the transmembrane protein (TM, amino acid 378-402) were used as antigens in microtiter plate enzyme immunoassays (EIA) to detect and discriminate antibodies to HTLV-I and II. The ENV-I and ENV-II peptide EIAs were able to correctly discriminate HTLV-I and II infections in 17 of 18 subjects whose infections were determined by a gene amplification method. Sera from 100 of 107 subjects with serologically confirmed infection with HTLV-I/II and 0 of 218 seronegative controls reacted with one or more of the peptides (sensitivity, 93.5%; specificity, 100%). Ninety-six of the 100 peptide positive sera reacted exclusively with either the ENV-I or the ENV-II peptide, thereby differentiating the two viral infections. The pattern of reactivity to the ENV peptides was distinct in different populations. Patients attending an Emergency Department, who had a history of drug abuse, and male inmate entering a correctional facility only had antibody reactivity to the ENV-II peptide. Subjects from Haiti and patients with HTLV-associated neurological disease only had antibody reactivity to the ENV-I peptide. Peptide-based enzyme immunoassays that distinguish antibodies to HTLV-I and HTLV-II will facilitate studies of the epidemiology of HTLV.