Pharmacokinetic and clinical studies of aztreonam (AZT) in perinatal infections in the field of obstetrics and gynecology were performed with the following results. 1. At one shot intravenous injection, 1 g AZT showed rapid distribution to the umbilical-cord serum with concentrations higher than 15 micrograms/ml in 1 hour 36 minutes after injection and higher 10 micrograms/ml even in 4 hours 30 minutes after injection. Significant difference in concentrations was not observed between arterial serum sample and venous serum sample of the umbilical-cord in a single subject. The concentration in the amniotic fluid reached a level higher 10 micrograms/ml in 3 hours 37 minutes after injection. 2. Distribution into milk reached a concentration between less than 0.4 micrograms/ml to 1.0 micrograms/ml by 6 hours after administration. 3. AZT 1 g x 2/day was given by intravenous drip infusion to 4 cases of perinatal infection in obstetrics and gynecology for 5 to 9 days. Clinically, AZT was effective for all the cases. Neither side effect nor abnormal laboratory value was observed. Consequently, AZT was considered to be highly effective and safe for its clinical use in the parturition and the puerperium.