Ampicillin administration during pregnancy exerted no toxic effect on the developing fetus. However, it decreased the body weight of the progeny and the weight of the placenta. Oxacillin had an adverse effect on the pregnancy, labor, fetus and newborn. It is inferred that ampicillin can be used for treatment purposes at any time of the pregnancy whilst oxacillin administration might involve a high risk for the mother, fetus and newborn.