Frozen samples from 23 low grade (grade I and II) astrocytomas were studied by means of a panel of monoclonal antibodies to macrophages, lymphocytes (and their subsets) and HLA-DR antigens. Macrophages were present in low to moderate numbers in 38%-86% of cases, the variance in figures depending on the antibody used. T lymphocytes, the majority of CD8 phenotype, were detected in low numbers in 78% of tumours. B lymphocytes were scanty in 22% (5/22) and totally absent in the remaining cases. HLA-DR antigen was expressed by tumour cells in 35% (6/17) of cases. These findings indicate that in some low grade astrocytomas there is a mononuclear cell infiltrate with macrophages and secondarily CD8+ lymphocytes playing the major role. The significance of these findings remains speculative at present.