Perforin-deficient [perforin (-/-)] mice were infected with two strains of JHM virus (JHMV) to analyze the role of perforin-mediated cytotoxicity in acute lethal and subacute central nervous system (CNS) infections. During both acute and subacute infections, the overall mortality of the perforin (-/-) mice was not different from that of the controls. Perforin (-/-) mice survived longer than the controls, consistent with reduced morbidity. Both strains of virus were cleared from the perforin (-/-) mice as in the controls; however, the rate of clearance was delayed in the perforin (-/-) mice, indicating that perforin-mediated cytolysis is involved in viral clearance. The absence of perforin-mediated cytolysis did not prevent encephalomyelitis or extensive demyelination. Cells undergoing apoptosis were detected in the CNS of both the perforin (-/-) and control groups, indicating that perforin is not essential for programmed cell death. Neutralizing antibodies were not detected in either group of mice until day 9 postinfection, when the majority of the virus had been cleared. These data further confirm the importance of cell-mediated cytotoxicity and suggest that additional components of the immune response contribute to the clearance of JHMV from the CNS.