HLA-DR is a class II cell surface glycoprotein of the human histocompatibility complex usually expressed on the surface of cells that are simultaneously presenting foreign antigen to T-lymphocytes. Using immunohistochemical procedures with two specific monoclonal antibodies to HLA-DR, HLA-DR-positive reactive microglia were found in gray matter throughout the cortex of postmortem brains of patients with senile dementia of the Alzheimer type (SDAT) and were particularly concentrated in the areas of senile plaque formation. Double immunostaining with antibodies to glial fibrillary acidic protein (GFAP) showed that the HLA-DR-positive cells were different from the reactive astrocytes although the occasional positively staining giant astrocyte was also seen. Small numbers of resting microglia were HLA-DR-positive in white matter of both normal and SDAT brains. The SDAT cases also had reduced cortical choline acetyltransferase (ChAT) levels. In the 11 brains studied, the number of hippocampal HLA-DR-positive cells was positively correlated with the numbers of plaques and negatively correlated with average cortical ChAT.