The purpose of the present study was to define the immunogenicity of two transplantable rat gliomas, designated F98 and D74, and to relate this to the phenotype and functional activity of tumor-infiltrating lymphocytes (TIL). Fischer rats, immunized with irradiated F98 tumor cells and challenged with intracerebral implants of ten F98 cells, had a median survival time of 49 days compared to 36 days for nonimmunized controls. In contrast, no statistically significant increases in survival times were noted in animals similarly immunized and challenged with the D74 tumors. No in vivo protection could be demonstrated in animals immunized and cross-challenged with either F98 or D74 glioma cells. Lymph node lymphocytes and TIL, isolated from animals immunized and challenged with F98 cells, were more cytolytically active than effector cells obtained from D74-immunized animals. Phenotypes of TIL isolated from intracerebral F98 gliomas of immunized rats were 52% OX-8+ and 21% W3/25+ compared to 31% OX-8+ and 19% W3/25+ for D74-immunized animals. Cytolytic activity against glioma targets was mediated by OX-8+ TIL, as determined by cell depletion experiments. Limiting dilution analysis showed that cytolytic T-lymphocyte precursors were present in TIL of F98 gliomas of immunized rats at a frequency of 1/3547 and were specific for F98 targets, while natural killer cell-like activity was low. Our data indicate that the F98 glioma was more immunogenic than the D74 glioma, as evidenced by increased numbers and activity of cytolytic effector cells and their precursors among TIL. This may explain in part the longer survival times observed in immunized animals challenged intracerebrally with the F98 gliomas compared to D74-immunized and -challenged hosts.