Ross River virus (strain T48) infection in mice causes an encephalomyelitis characterized by focal, primary demyelination in the cerebellum, brain stem, and spinal cord. Maximal serum and brain content of virus occurs on days 2 and 4, respectively. Virus is not detectable in serum after day 3 or in brain after day 9. Histopathological lesions are present by day 2 and consist of perivascular macrophage and polymorphonuclear leukocyte infiltration, focal necrosis in the internal granule cell layer, and myelin disruption. Mononuclear cell infiltrates are present by day 5. Foci of demyelination in the presence of preserved axons become more widespread by day 8, and early partial remyelination occurs by day 13. Immunosuppression reduces the mononuclear cell infiltration but does not alter the demyelination . Although the mechanism of Ross River virus-induced demyelination is not known, these findings suggest that it is not immune mediated.