The technique of in vivo depletion with T cell subset-specific monoclonal antibodies was used to study the involvement of CD8+T cells in protection/pathogenesis during the acute and chronic demyelinating phases of Theiler's murine encephalomyelitis virus (TMEV)-induced disease. Mice rendered CD8-deficient prior to infection with TMEV were less efficient at clearing virus from the central nervous system compared to intact animals and also suffered demyelinating disease of earlier onset and increased severity. This indicates that CD8+ cells have a protective role in virus clearance at early times post-infection, and may also be involved in downregulating the severity of the chronic demyelinating disease. How CD8+ T cells function to produce these effects is discussed.