A subpopulation of tumor-infiltrating lymphocytes (TILs) and the expression of major histocompatibility complex (MHC) antigens of the tumor cells were examined in 14 glioma and 13 metastatic brain tumor tissues. In both tumors, most of the TILs were T lymphocytes, and both phenotypes of the cytotoxic/suppressor and helper/inducer T lymphocyte were found. On examination of MHC antigens, beta 2-microglobulin was shown intensely on tumor cells in all cases, and the monomorphic determinant of the human leukocyte antigen (HLA-DR) was shown in 10 glioma and in 5 metastatic cases. The correlation between the number of TILs and MHC antigen expression on tumor cells was equivocal as a whole in cases of both glioma and metastatic brain tumor.