The antigenic properties of leukemic cells from five patients with adult T-cell leukemia were studied with rabbit anti-MOLT-4 and anti-human thymocyte antisera using indirect membrane immunofluorescent staining. The E rosette-positive, surface immunoglobulin (sIg) negative leukemic cells from these patients gave a positive reaction with appropriately absorbed antisera, which reacted specifically with thymocytes, cells from T-cell acute lymphoblastic leukemia (T-ALL) and T-ALL derived lymphoblastoid cell lines (T-LCLs) and normal fresh peripheral T-cells. Nevertheless, antisera further absorbed with fresh peripheral blood lymphocytes (FN-PBL) lost almost all reactivity with the leukemic cells as well as with normal peripheral blood T-cells but still retained reactivity with thymocytes, T-LCLs and T-ALL cells. The results suggest that adult T-cell leukemic cells possess a mature T-cell antigen but not a thymus-specific antigen.