To define the antibody profile of early seroconversion in infection with human T-cell lymphotropic virus type I (HTLV-I), consecutive serum samples from 10 subjects presumed to have seroconverted on the basis of the particle agglutination test were studied by three enzyme immunoassays and two confirmatory tests (radioimmunoprecipitation and western blot). 3 samples positive and 1 sample indeterminate in the confirmatory tests were reactive in one enzyme immunoassay, which used recombinant envelope antigen, but not in the other two enzyme immunoassays. 2 of 38 particle-agglutination-negative samples had a prozone effect. The confirmatory tests identified 8 seroconverters (7 women, 1 man); their serum samples were used to study the antibody reactivity by western blot assays to HTLV-I specific antigens (three recombinant proteins spanning the N-terminal, middle, and C-terminal env glycoprotein gp46; a recombinant transmembrane protein gp21; a recombinant tax protein; and three gag proteins [p28, p24, and p19]). All 8 seroconverters had antibody reactivities to the C-terminal region (aminoacid residues 229-308) of gp46 and to gag p19 and p24 when their seroconversion was detected.