Localization of beta-2 integrins in normal and Alzheimer disease temporal cortex was studied immunohistochemically. Resting microglia were found to express constitutively CD11a (LFA-1), CD11b (Mac-1, CR3), CD11c (P150, 95; CR4), and CD18 (beta-2). They were also found to express constitutively leukocyte common antigen and the immunoglobulin receptor Fc gamma RI. The intensity of expression of each of these antigens was enhanced on reactive microglia in Alzheimer disease tissue. HLA-DR was detected on only a few microglia in control tissue, but was intensely expressed on large numbers of reactive microglia in Alzheimer tissue. These data are consistent with a leukocyte origin and a phagocytic role for microglia. They provide further evidence of an inflammatory response of brain tissue in Alzheimer disease. The microglia were found to make up 9-12% of the total glial population in gray matter and 7.5-9% in white matter.