The haematological and immunological characteristics of 34 healthy anti-HTLV-I antibody-positive individuals (HTLV-I carriers) in southwestern Japan were examined. No significant difference was noted between carriers and the controls in counts of RBC, WBC and the absolute number of lymphocytes. The serum IgG in the carriers was higher than that of the controls. The percentages of OKT4, OKT8, OKIa1 and B1-positive cells were found to be normal in the peripheral blood of the carriers, whereas the percentages of OKT11 and anti-Tac-positive cells were significantly higher in the carriers than in the controls. A correlation was observed between the percentages of anti-Tac-positive cells and the titres of anti-HTLV-I antibody in the carriers. After a 72 h incubation of peripheral blood lymphocytes with medium alone, the percentage of anti-Tac-positive cells tended to decrease in the controls, but to increase in carriers, with the appearance of large blastoid cells resembling blastic transformed lymphocytes cultured with mitogen. Tac and Ia antigens were markedly expressed on these large blastoid cells.