Light microscopic and ultrastructural immunohistochemistry of cell adhesion molecules (CAMs) and major histocompatibility class II antigens (Ia) expression in experimental murine Toxoplasma encephalitis (TE) revealed a prominent upregulation of the intercellular cell adhesion molecule-1 (ICAM-1) and of Ia on cerebral endothelia, microglia, ependyma, and choroid plexus epithelium during acute and chronic TE. Microglia also expressed Mac-1 and leukocyte function-associated antigen-1 (LFA-1), which are both ligands of ICAM-1, as well as CD45. The prominent simultaneous expression of a multitude of these molecules on microglia is indicative of a central immunologic function of this cell type in TE. Additionally, occasional astrocytic processes slightly expressed Ia in full-blown TE. The vascular cell adhesion molecule-1 (VCAM-1) was restricted to endothelia of cerebral blood vessels, which frequently showed perivascular cuffing of inflammatory cells, ependyma, and choroid plexus epithelium. Upregulation of Ia, CAMs and their ligands correlated with disease activity. Immunohistochemical analysis of the functional state of infiltrating T cells showed a preferential recruitment of CD44+ memory and activated interleukin-2R+ T cells in TE.