Resistance of perforin knockout (PKO) mice to infection with Toxoplasma gondii was assessed in models of acute infection and during chronic disease. PKO mice vaccinated with the attenuated mutant, ts-4, displayed severely defective CTL responses against tachyzoite-infected targets. Lysis of the NK target, YAC-1, was also severely impaired in PKO mice following ts-4 vaccination. In contrast, wild-type mice developed high levels of CTL and NK lytic activity after ts-4 vaccination. Despite severely defective lytic activity, vaccinated PKO animals were completely resistant to challenge with the virulent strain RH, which normally causes a lethal acute infection. Resistance was attributable to production of IFN-gamma, which remained unimpaired in the PKO animals. In contrast, when PKO mice were infected with low virulence parasite strain ME49, which progresses to the cyst-forming stage after passage through an acute phase, accelerated mortality was observed beginning at 75 days postinfection. A three- to fourfold increase in brain cyst numbers was also found by day 30 in infected PKO animals. Nevertheless, the PKO strain produced normal levels of IFN-gamma after ME49 infection, ruling out impaired production of the latter cytokine as a cause of increased susceptibility. Together, these results show that perforin-dependent cytolytic function is not required for host resistance to lethal acute infection in preimmunized animals, but that the latter activity contributes to the control of infection during the chronic stage.