Following intraperitoneal infection of BALB/c mice with the A7(74) strain of Semliki Forest virus, the virus spreads to the central nervous system (CNS) and initiates an acute inflammatory reaction which includes lesions of primary demyelination. This demyelination is dependent upon activated T lymphocytes. To determine whether CD4+ or CD8+ T cells are involved in the pathogenesis of the demyelination, we have investigated the course of infection in animals treated with monoclonal anti-CD4 or anti-CD8 antibodies. In the normal course of infection, virus was detectable in the brain by infectivity assay and in situ hybridization for up to 14 days. Antiviral immunoglobulin M (IgM) and all subclasses of IgG were produced. From day 10 to 21 postinfection lesions of inflammatory demyelination were present, most notably in the cerebellum and corpus callosum but also in other white matter tracts. Administration of anti-CD4 antibodies removed CD4+ cells from the spleen, prevented production of antiviral IgG, increased virus titers in the brain, and increased demyelination. Administration of anti-CD8 antibodies depleted CD8+ cells from the spleen and did not affect antiviral IgG synthesis or spread of brain virus but reduced CNS inflammatory responses and virtually abolished lesions of demyelination. Administration of both antibodies depleted both T-cell subsets from the spleen, prevented IgG antibody production, increased brain virus, and abrogated both CNS inflammation and lesions of demyelination. In conclusion, the CNS demyelination induced by Semliki Forest virus can be prevented by in vivo depletion of CD8+ T lymphocytes.