Tissue sections of brains from patients with multiple sclerosis (MS) and from control individuals were immunostained with MHC class II and glial or vascular endothelial cell antibodies and analyzed by confocal microscopy. MHC class II was abundant in and around actively demyelinating MS lesions and was detected on microglia, phagocytic macrophages, and perivascular macrophages. Astrocytes and vascular endothelial cells were MHC class II-negative. Changes in the size and shape of MHC class II-positive cells associated with MS lesions suggest that microglia transform into phagocytic macrophages, and that they are actively involved in demyelination. Many MHC class II-positive perivascular macrophages within MS lesions contained abundant intracellular MHC class II immunoreactivity; these cells may be involved in antigen presentation and in T cell activation.