We have examined expression of antigens defined by HT462 monoclonal antibody (mAb), together with other HTLV-I related antigens using phorbol 12-myristate 13-acetate treated leukemic mature T cells. Thirteen patients with adult T-cell leukemia (ATL), 3 patients in remission states of ATL and 5 patients with non-ATL were examined. All ATL cells expressed the HT462 antigen, however cells from patients in remission did not express the HT462 antigen. A low percentage of cells from 2 out of 5 patients with non-ATL mature leukemic T cells expressed the HT462 antigen, although these cells did not express other HTLV-I related antigens. Cells of HTLV-I infected human cell lines expressed the HT462 antigen. Three HTLV-I infected rat cell lines (TARS-1, TART-1, TARL-2) did not express the HT462 antigen, although cells of these lines expressed other HTLV-I related antigens. Characterization of the HT462 antigen by strip radioimmunoassay based on western blotting technique using cell lysates of HUT102 cells revealed two additional bands (p68, p35) together with previously reported proteins (gp52, p42). Only p68 was seen in western blots using cell lysates of the rat cell lines. These findings further suggest that the HT462 antigen is a cellular component induced in virus transformed human cells and not a virus encoded protein.