Mouse hepatitis virus, strain JHM (MHV-JHM), causes a late onset, clinically apparent, demyelinating encephalomyelitis in 40% of suckling C57BL/6 mice born to immunized dams. Suckling mice born to unimmunized dams rapidly succumb to an acute encephalomyelitis. MHV-JHM can be isolated from the brains and spinal cords of maternal antibody-protected mice when the late onset disease becomes clinically apparent, showing that the virus must be present in these mice when they are still asymptomatic. To determine which cells of the central nervous system (CNS) were potential reservoirs for the virus during the asymptomatic period, tissue sections were assayed simultaneously by immunoperoxidase and immunofluorescence staining for the presence of viral antigen and for glial fibrillary acidic protein (GFAP), a marker for astrocytes. The results indicate that 20% (range 0-52%) of the MHV-JHM infected cells in asymptomatic mice were astrocytes. In mice symptomatic with late onset hindlimb paralysis, a higher percentage of infected cells were astrocytes. These results indicate that astrocytes are a target cell in both symptomatic and asymptomatic mice persistently infected with MHV-JHM, and suggest that the astrocyte is a potential cellular reservoir for MHV-JHM in asymptomatic mice.