The temporal activities of suppressor T lymphocytes (Ts) and cytotoxic T lymphocytes (CTL) were investigated in a syngeneic murine malignant glioma (a methylcholanthrene-induced ependymoblastoma of C57BL/6 mouse origin, 203-glioma). After the s.c. tumor inoculation, it was suggested that both Ts and CTL were generated with target specificity against 203-glioma cells, because neither Ts nor CTL activity were seen against syngeneic EL 4 (benzpyrene-induced thymoma), allogeneic P815 (methylcholanthrene-induced mastocytoma of DBA/2 mouse origin) or YAC-1 (Moloney leukemia-induced T-cell lymphoma of A/Sn mouse origin), but only against 203-glioma. It was found that the generation of Ts preceded that of CTL and that the turnover was faster; furthermore, Ts were generated in the thymus and spleen, while CTL were distributed in regional lymph nodes and spleen. Surface marker analysis revealed that only Lyt-1-.2.3+ T-cells participated in suppressor responses in contrast to both Lyt-1-.2.3+ and Lyt-1+.2.3+ T-cells participating in cytotoxic responses. The effects of adult thymectomy (ATx) on the changes of the immunized T-cell subsets were also investigated. In mice thymectomized 3 weeks previously, the Ts activity was abrogated, whereas the CTL activity increased markedly and Lyt-1+.2.3+ T-cells were not detected. The results suggest that CTL or their precursors bearing Lyt-1+.2.3+ phenotype and Ts bearing Lyt-1-.2.3+ phenotype are short-lived lymphocytes. Accordingly, it is suggested that in tumor-bearing mice short-lived Ts are generated earliest with target specificity and, due to the reciprocal relationships between Ts and CTL activities, may have a modulating influence on CTL; furthermore, ATx may alter the patterns of generation of the precursor T-cells and Ts.