Trypsin inhibitor isolated from Ascaris lumbricoides suis and injected into pregnant BALB/c mice disturbed prenatal development of mice fetuses. The nature and intensity of prenatal disturbances in development are determined by the trypsin inhibitor dose. The symptoms that occurred after administration of the highest doses of Ascaris trypsin inhibitor to pregnant mice included: decreased body weight gain (p<0.05) as compared to controls, bleeding from the uterus, abortions and mortality. All doses of trypsin inhibitor exhibited an embryotoxic effect. Trypsin inhibitor significantly decreased the number of fetuses per litter, increased the frequency of litter resorption, produced a delay in bone formation and included pathological changes. Teratogenic effects of trypsin inhibitor were apparent in fetuses after doses of 200 - 400 mg per kg. The most frequent developmental defects were: cleft palate, fusion of ribs and micrognathia. No defects were noted in control groups. The trypsin inhibitor from Ascaris is a substance of considerable biological activity, which may be important in the outcome of the host-parasite relationship. It may be also a parasite pathogenic factor.