Forty-five nervous system tumors (9 glioblastomas, 9 meningiomas, 15 assorted primary neural tumors including 3 medulloblastomas, and 12 brain tumors metastatic to the brain were analyzed for their content of lymphocytes, granulocytes, and macrophages. Cell suspensions were prepared by enzymatic digestion; lymphocytes and granulocytes were quantitated by morphology following cytocentrifugation, and macrophages were quantitated by IgG EAC (erythrocyte-antibody-complement) rosette formation. EA (erythrocyte-antibody) adsorption to sections of tumor was employed to determine the distribution of the IgG Fc receptor-positive cells within the tumors and to serve as quality control for selective release of Fc receptor-positive cells by enzyme digestion. The 9 glioblastomas had a mean macrophage content of 41% (range: 5–78%); the 9 meningiomas, 42% (range: 5–80%); the 3 medulloblastomas, 6% (range: 2–15%); and the metastatic tumors, 21% (range: 2–50%). Lymphocyte contents were variable but generally less than 10%. Most tumors contained less than 10% granulocytes. EA adsorption demonstrated that Fc receptor-positive cells were distributed throughout the tumor mass, although different types of patterns were observed. There was an excellent correlation between the percent EAC rosettepositive cells in suspensions and the extent of EA adsorption to the tumor sections. The significance of the study primarily rests in the demonstration that most nervous system tumors contain high numbers of infiltrating host cells, primarily macrophages.