We investigated the mechanism by which mice immunized with an adenovirus-herpes simplex glycoprotein B recombinant (Ad-HSV.gB) are protected from challenge with a vaccinia (Vac)-HSV.gB and the cognate virus, HSV. C57/BL mice immunized intraperitoneally with Ad-HSV.gB were protected against an intracerebrally inoculated lethal dose of a Vac-HSV.gB or HSV-1. CD8+ cytotoxic T lymphocytes but not interferon-gamma-dependent mechanisms play a major role in the clearance of both viruses from the central nervous system. These results demonstrate that the administration of two recombinant viruses carrying the same foreign insert for either immunization or challenge in a mouse protection model provides useful information about the protective nature of the inserted gene product.