Antimouse testis antisera (AMTAS) were prepared in rabbits by the injection of testis extracts of the adult mice. Administration of 35.6 mg of AMTAS into pregnant mice on day 12.5 of gestation resulted in 27.3% fetal malformations, while administration of 17.8 mg of the same antisera induced 6.0% malformations. Malformations such as limb anomalies, cleft palate, omphalocele, and spina bifida were often accompanied by hemorrhagic lesions. No malformations associated with hemorrhage were found in the control group in spite of the occurrence of hematomas. The immunofluorescent analysis revealed the localization of antitestis antibody (ATAb) in the basement membrane of the testis, in the spermatozoa, the mesometrial vessels, trophectroderm, and the Reichert's membrane. In contrast, there was no localization of IgG when normal rabbit sera were used. The results suggest that the antibody against the basement membrane or against the spermatozoa may be responsible for the occurrence of congenital malformations in the pregnant mice injected with rabbit AMTAS.