AbstractActinomycin D (act D) was teratogenic to Syrian hamsters. The most effective dosage, 200 μg/kg of body weight, produced malformations when injected on day 4, 6, 8 or 10 of gestation. 100 μg/kg produced malformations only after injections on day 6 or 8. 300 μg/kg was strongly embryolethal, but some malformed fetuses were found among live survivors. 400 μg/kg caused 100% maternal lethality. The teratogenicity and fetal and maternal lethality were reduced when act D was mixed with nucleic acids. DNA was more potent than RNA in preventing toxic effects of act D. The embryolethal effect of act D was also diminished by treatment with a combination of 4 mg progesterone and 1 μg of estrone, although maternal lethality and teratogenicity were not preve