The fundamental study of cefmetazole treatment in a clinical situation during the perinatal period were made, and the following results were obtained. Absorption of CMZ in pregnant women is rapid; and the intravenous injection intravenous drip infusion and intramuscular injection of CMZ all reached their peak serum levels in a short time with a half-life of approximately 1 hour or so. The transference of CMZ to the fetus through placenta is good. It is so good that a single dose of 0.5 approximately equal to 1.0 g by intravenous injection, intravenous drip infusion or intramuscular injection can obtain desired concentration levels covering MIC of main causative pathogenic organisms in the cord blood, amniotic fluid and blood in the fetus. Therefore, the administration of CMZ with this dose and method of administration once or twice daily can prevent or treat intrauterine infection. The transition of CMZ to mother's milk is a small quantity and the transition of CMZ through mother's milk to a neonate is considered to be a very small amount. The absorption of CMZ in neonate is rapid. It reaches a peak serum concentration 15 minutes after the intravenous injection. The half-life varies depending on the number days after birth. With the administration of 20 mg/Kg, the half-life at 0 day after birth is 6.32 approximately 6.78 hours, 3.79 hours at 1st day after birth and 2.27 approximately 2.72 hours at 5th-6th day after birth. Excretion into the urine is slow on 0 day after birth and becomes rapid on the 6th day after birth, coinciding with the maintained level of CMZ in the blood. Positive results were obtained from the overview of CMZ administration in the prophpylaxis or treatment of intrauterine infection during the perinatal period. No abnormalities were noted from the examinations performed on a neonate delivered from a mother who had received CMZ during perinatal period. Judging from the various results mentioned above, 20 mg/kg of CMZ administered to a neonate twice a day at 12 hour intervals would be sufficient to attain therapeutic efficacy.