The eradication of infectious virus from the central nervous system (CNS) following infection with the neurotropic JHM strain of mouse hepatitis virus (JHMV) is thought to be immune-mediated. Furthermore, a significant decrease of infectious virus coincides with the appearance of prominent inflammatory infiltrates in the brain and spinal cord. In the present study, mononuclear cells infiltrating the brain during JHMV infection were isolated and characterized. While all subsets of immune cells were present, there appeared to be a temporal relationship between the peak incidence of CD8 + T cells (40% of total isolated cells) and reduction of virus at day 7 post-infection. Cells with the natural killer (NK) phenotype (at least 30%) were also present throughout infection. These data suggest that CD8 + T cells and NK cells are prominent among cells which infiltrate the brain during JHM virus infection and may have important roles in reduction of virus within the CNS.