Infections with 4 strains of Toxoplasma gondii were studied in mice and hamsters. In mice, infections with strains RH, T-1, and T-45 were always fatal, but strain ts-4 infection was not (ie, the mice survived). In hamsters, the RH strain always gave rise to a fatal infection and the T-1 strain killed about 90% of the hamsters, but as many as 10(6) tachyzoites of the T-45 and ts-4 strains failed to cause fatal infection. The lesions of the ts-4 infection in hamsters were limited to mononuclear inflammation and a scar at the injection site. Also, only the ts-4 strain did not produce chronic infection in hamsters. To evaluate degrees of pathogenicity of the strains, percentages of mortality, survival time, and such subtle changes as fever and weight loss can be used.