The humoral immune response in sera from 30 human T cell lymphotropic virus type I (HTLV-I)-positive individuals from Martinique in the French West Indies was studied. The subjects were subdivided into those suffering from TSP/HAM and those being asymptomatic. In general, TSP/HAM patient sera seemed to contain more virus-specific antibodies than did the sera from the asymptomatic subjects. Three of the 13 TSP/HAM sera and 1 of the 17 asymptomatic sera contained HTLV-I-specific IgM antibodies, whereas 6 and 5 sera, respectively, contained IgA antibodies. By correlating the ability of patient sera to inhibit HTLV-I-induced syncytia with their antibody reactivity in ELISA to 42 synthetic peptides, together corresponding to the entire envelope glycoprotein of HTLV-I, a number of putative neutralizing domains were identified. Eight synthetic peptides representing the regions with the highest coefficient of correlation between neutralizing titer and ELISA reactivity were employed to specifically adsorb potentially neutralizing antibodies, and were also used directly, without sera, in the syncytium-neutralizing test. By those techniques, three novel and two previously described domains that seemed to contain neutralizing epitopes were identified. Two of the novel neutralizing sites resided in the external glycoprotein (gp46) and were contained within amino acids 53-75 and 287-311, respectively, and one was located in the transmembrane glycoprotein (gp21) within amino acids 346-368. Our findings may have implications for the rational design of subunit vaccines for prevention of and/or alteration of the clinical outcome of HTLV-I-related diseases.