Glial cells of the central nervous system (CNS) are postulated to function as immune accessory cells which may regulate immune reactivity occurring within the CNS, activating or alternatively inhibiting T cell responses. We have utilized surgically resected cerebral tissue derived from young adult humans to prepare dissociated cultures of glial cells (mixed astrocyte-microglia-oligodendrocyte cultures) and demonstrate that such cells are capable of acting as stimulators of primary T cell responses, using proliferation of T cells to allogeneic determinants on the glial cells as the test system. Studies of resected adult cerebral tissue indicated major histocompatibility complex (MHC) class II antigen expression on microglia in situ. Using a mixed lymphocyte reaction (MLR), we observed that enriched microglial cultures alone were capable of stimulating primary responses of freshly isolated T cells or the CD4+ T cell subset, a response which could be inhibited with an anti-MHC class II blocking antibody. In agreement with previous studies using rodent-derived astrocytes, we found that human astrocytes (fetal), could not initiate a primary T cell response even after up-regulation of MHC class II antigen expression with interferon gamma (IFN gamma) and tumor necrosis factor alpha (TNF alpha). Our results indicate that a primary T cell response, as well as a secondary response to a recall antigen, can occur within the CNS; our data implicate microglia as the central cell involved in the former.