Immunity to toxoplasmosis was investigated in outbred pigs after feeding Toxoplasma gondii oocysts inactivated by irradiation. In 2 experiments 23, 2-3-mo-old pigs were fed (12 pigs only once and 11 pigs twice) approximately 10(5) oocysts irradiated at 0.3 or 0.4 kGy of 137Cs and 11 pigs served as unvaccinated controls. Eleven to 13 wk later, 15 vaccinated pigs and 5 nonvaccinated pigs were challenged orally with a high dose (10(5) or 10(6)) of oocysts, and 7 vaccinated and 6 nonvaccinated pigs were challenged orally with a low dose (10(2) or 10(3)) of oocysts; 1 vaccinated pig was not challenged. All nonvaccinated pigs challenged with a high dose became ill starting 4-5 days postchallenge (p.c.). One pig died day 9 p.c., 1 died day 16 p.c., and 1 was killed day 10 p.c. because of weakness. All vaccinated pigs, nonvaccinated pigs challenged with a low dose, and the 1 vaccinated unchallenged pig remained clinically normal. Bioassay of pig tissues (tongue, diaphragm, brain) in mice indicated fewer tissue cysts in tissues of vaccinated pigs compared with unvaccinated pigs following challenge with live oocysts. However, vaccination with 1 or 2 doses of irradiated oocysts did not prevent formation of tissue cysts even in pigs challenged with 10(2) oocysts.