TdT and surface T-cell antigens of leukemic cells from 14 patients with ATL were analysed with a TdT immunofluorescent assay kit(BRL) and monoclonal antibodies (Becton-Dickinson, Leu-1, Leu-2a and Leu-3a antibodies). The TdT assay kit and the monoclonal antibodies were tested to confirm their specificities using various types of cultured cell lines and fresh hematopoietic malignant cells. The reactivities of the kit and antibodies were consistent with the previously reported specificities. TdT activity in leukemic cells from 6 out of 7 ATL patients was not detected; however, cells from one patient did contain weak TdT activity. The majority of the ATL cells reacted with Leu-1 and Leu-3a antibodies but not with Leu-2a. The lack of or weak TdT activity and the expression of Leu-1 and Leu-3a reactive T-cell antigens of ATL cells indicate that ATL cells are of mature and helper/inducer T-cell origin.