We studied immunological functions of peripheral blood lymphocytes (PBL) from human T-cell leukemia virus type I (HTLV-I)-seropositive healthy carriers in vitro. Proliferative responses of PBL to T-cell and B-cell mitogens such as concanavalin A (Con A), pokeweed mitogen (PWM), and Staphylococcus aureus Cowan I (SAC) were moderately impaired in HTLV-I carriers compared with normal controls. Immunoglobulin (Ig)-producing activity of PBL stimulated with B-cell mitogens were also impaired in HTLV-I carriers. However, cytotoxic T-cell activity induced by in vitro culture was not impaired but slightly increased in HTLV-I carriers. Natural killer-cell activity was only slightly decreased. By a flow cytofluorometric analysis of the cell surface phenotypes of PBL, the percentage and the mean fluorescence intensity (MFI) of CD 3-positive cells and CD 4-positive cells were significantly decreased in HTLV-I carriers. The percentage and the MFI of CD 8-positive cells was not changed. The percentage and the MFI of CD 25-positive cells were increased. These results suggest that some immunological abnormalities are already present in HTLV-I carriers and such abnormalities have some roles for the leukemogenesis from the infection of the HTLV-I into adult T-cell leukemia (ATL).