Detection of HLA-DR, a class II major histocompatibility antigen, on glial cells is dependent not only on duration and type of tissue fixation and processing, but also on clinical factors. Glial cells labeled by anti-HLA-DR were consistent with microglia by light microscopic and ultrastructural criteria, and were colabeled with other microglial markers, including LN-1, Leu-M5, and leukocyte common antigen (LCA). In young and elderly subjects who died suddenly, anti-HLA-DR labeled microglia in the white matter, but far fewer cells in the gray matter. In subjects who died of chronic debilitating illness, such as Alzheimer's disease and carcinomatosis, anti-HLA-DR labeled numerous microglia throughout both the gray and white matter. In Alzheimer's disease, microglia were aggregated in compact senile plaques, but loosely associated with diffuse amyloid deposits. These results suggest that HLA-DR may be constitutively expressed in white matter, but induced in gray matter microglia in chronic disease states or in association with amyloid deposits.