Sera from patients with adult T-cell leukemia (ATL) or other diseases and from healthy adults, whose titers of antibodies against ATL-associated antigens (ATLA) had been determined by indirect immunofluorescence, were analysed by a procedure of immunoprecipitation followed by SDS-polyacrylamide gel electrophoresis. For this an ATL virus (ATLV)-producer cell line, MT-2, was labelled with [35S]-methionine. All 12 anti-ATLA-positive sera but none of the eight anti-ATLA-negative sera tested reacted specifically with four polypeptides with molecular weights of 70,000, 53,000, 36,000 and 24,000 daltons. Furthermore, enrichment of three polypeptides with molecular weights of 76,000, 43,000 and 28,000 daltons was observed on reaction with anti-ATLA-positive sera. In control experiments using ATLA-negative T-cell lines, Molt-4 and HPB-ALL, none of these seven polypeptides were precipitated by reaction with anti-ATLA-positive sera. All six anti-ATLA-positive sera tested were shown to react with a polypeptide with a molecular weight of 24,000 of purified ATLV.