Trophozoites from the peritoneal cavities of mice infected with the RH strain Toxoplasma gondii were given irradiation in doses of 5, 10, 15, and 20 kiloroentgens (kr). CD-1 strain mice that received intraperitoneal inoculation of trophozoites irradiated with 5 kr all died of toxoplasmosis, but the mice that received trophozoites irradiated with the higher doses all survived. The survirors that were examined were found to be free of toxoplasmic cysts. Single doses of these irradiated vaccines provided good protection to subsequent virulent challenge. This protection was 100% in the first 3 weeks after the immunization. Survivors of the first challenge were also solidly protected against a subsequent rechallenge.