Multinucleated giant cells that are characteristic of the encephalomyelopathy of acquired immunodeficiency syndrome are stained by a lectin histochemical method. The particular lectin used, Ricinus communis agglutinin I, has been shown to bind to endogenous microglial cells in human brain, as well as to endothelial cells of blood vessels. These results suggest that multinucleated giant cells in the brain of patients with acquired immunodeficiency syndrome may be derived from endogenous microglia.