Sixty-five malignant gliomas (astrocytomas grade 3 and 4 and glioblastomas) were examined by means of immunoperoxidase staining on frozen tissue using various monoclonal antibodies directed against macrophages, lymphocytes and natural killer cells. Depending on the antibody used, the presence of macrophages in tumours ranged from 85%–100%. Many of the tumours contained substantial numbers of macrophages not only, as expected, in necrotic areas but also in intact tumour tissue. Eighty-nine percent of 39 tumours tested contained Fc receptorbearing mononuclear cells in viable tumour. In 100% of 44 tumours tested for HLADR class 2 major histocompatibility complex antigen this antigen was detected in the macrophages. In 40% of these 44 cases, HLADR antigen was also present on the tumour cells. Eighty-eight percent of 53 tumours tested contained T cells in viable tumour and the majority of these cells were T cytotoxic/suppressor (T8). Twenty-four percent of 33 tumours contained no T helper/inducer (T4) lymphocytes and in the other 76% there were few positive cells. Only 9% of 21 tumours contained natural killer cells (NK). B cells were absent from 88% of 61 tumours and almost all of the remainder contained only a small number of B cells. The findings are discussed with reference to a possible host immune response to gliomas and relevant literature is reviewed.