Immunological responses to an experimental brain tumor of mice [the 20-methylcholanthrene-induced malignant glioma, 203-glioma)] were investigated. The killer T-cell activity of spleen cells, which was specific against 203-glioma cells, began to be severely impaired 2 weeks after intracranial inoculation; this impairment was concurrent with increased intracranial pressure, which was due to developing tumor growth. On the other hand, the killer T-cell activity continued for over 4 weeks in mice inoculated with the mitomycin C-treated tumor cells. Surface marker analysis showed that Lyt-1-2,3+ killer T-cells were predominant in intracranial tumor-bearing mice, whereas both Lyt-1-,2,3+ and Lyt-1+,2,3+ killer T-cells were equally present in s.c. tumor-bearing mice. The effects of adult thymectomy on the immune responses against 203-glioma were also investigated in intracranial and s.c. tumor-bearing mice. In both the intracranially and s.c. inoculated groups, killer T-cell activity was increased in mice thymectomized before 3 weeks and decreased in mice thymectomized before 10 weeks. In these mice, Lyt-1+,2,3+ killer T-cells were not detected, which suggests strongly that the progenitors of Lyt-1+,2,3+ killer T-cells are short-lived lymphocytes in contrast to those of Lyt-1-,2,3+ killer T-cells, which survive more than 10 weeks after adult thymectomy.