Sera from HTLV-I carriers were studied with immunoprecipitation using [35S]-cysteine labelled HUT 102 cell lysate as an antigen and were analyzed further by SDS-polyacrylamide gel electrophoresis.The viral proteins which specifically reacted with the the sera from carriers were mainly gp61, p55, gp45, p42, p24 and p19. Analyzing the thickness of bands on electropherograms, the intensity of antibody reaction to the above proteins was strongest in gp61 and in decreasing order of gp45, p55, p24 and p19.Secondly the incidence of detecting the antibodies to each viral protein was studied. In the lowtiter sera, the antibodies to envelope protein such as gp61 and gp45 were mainly detected, while the antibodies to core proteins such as p55, p24 and p19 were found in the high-titer sera. The antibody to gp61 was demonstrated in almost all the samples, and the thickness of this band was well correlated with the antibody titer of the sera.p42 is encoded by pX region of HTLV-I gene. The thickness of the band of anti-p42 antibody and the incidence of detecting this antibody had no relation to the antibody titer in contrast to the above proteins. This observation suggests that the mechanism and process of anti-p42 antibody production appears to be different from those of other proteins of this virus.