After sensitization by M-protein fraction (MP) of hemolytic streptococci for two generations, Wistar rats were injected with 9 ml/kg of MP on day 9 of gestation (plug day = day 0). The incidence of malformations was about 12% whether anti-MP antibody was positive or not in the maternal sera. The primary malformations were ventricular septal defect, anophthalmia, and microphthalmia. Light microscopic study revealed no myocarditis nor endocarditis in either the maternal or fetal heart. No antinuclear antibody was detected in the maternal sera by the fluorescent antibody technique. No complete heart block was found in the fetus by electrocardiogram. These results were similar to previous findings using a single injection of MP, and therefore further demonstration that the principal factor in the teratogenesis induced by MP administration in the rat is not an inflammation in the fetal organs caused by an autoimmune mechanism but rather yolk sac dysfunction.