We have studied frozen tissue from 19 oligodendrogliomas with a panel of antibodies to lymphocytes and their subsets, macrophages, natural killer cells, and HLA-Dr antigens. Macrophages were detected in moderate numbers in 60%-100% of tumors depending on the antibody used. T lymphocytes were fewer in number than macrophages and were present in 62% of cases. Most of the T lymphocytes were of the CD8 phenotype. CD4 lymphocytes were very few in number and present in only 18%. B cells and natural killer cells were absent from all cases. HLA-Dr antigens were expressed by macrophages in all cases but never on tumor cells. The implications of these findings are that macrophages and, to a lesser extent, CD8 lymphocytes are the predominant cells infiltrating oligodendrogliomas and that they may exert cellular immune functions.