Although T cells play a pathogenetic role in MS, specific disease-inducing T cells have not been identified. T cells can be labeled and traced in adoptively transferred experimental autoimmune encephalomyelitis (EAE), a T-cell-mediated animal model for MS. We have followed the appearance and topographic localization of radiolabeled myelin basic protein-reactive (MBP+) T cells in evolving lesions as EAE extended to other regions of the CNS. By high-resolution autoradiography, we confirmed that MBP+ cells initially homed to perivascular regions in the lower spinal cord. With increasing time after cell transfer, labeled cells appeared in more recent lesions in rostral locations (upper spinal cord, cerebellum, and forebrain) and constituted a progressively smaller percentage of cells in lower spinal cord lesions. The presence of unlabeled inflammatory cells in the CNS parenchyma coincided temporally with clinical signs. In agreement with previous studies, we have shown that MBP+ cells constituted a minority (mean, <1.5%) of the total infiltrating cells and were most numerous in fresh lesions. We suggest that the perivascular regions of recent lesions would be the most likely areas to detect putative antigen-specific cells in MS lesions.