Pharmacokinetics and clinical studies on an injectable monobactam antibiotic aztreonam (AZT), were carried out in perinatal infections in obstetrics and gynecology and the obtained results are summarized as follows. 1. Pharmacokinetic study (1) Upon one-shot intravenous injection of AZT 1 g before delivery, maternal serum concentration of AZT was 89.0 micrograms/ml immediately after the injection and a half-life (T 1/2) of 0.96 hour was observed. Umbilical-cord serum concentration showed a peak value of 16.5 micrograms/ml at 1.26 hours after the injection and gradually decreased with a T 1/2 of 1.91 hours. The transfer into amniotic fluid was observed and the peak value of AZT in amniotic fluid reached 12.9 micrograms/ml at 5.57 hours after the injection and slowly decreased thereafter with a T 1/2 of 4.42 hours. Transfer and disappearance in one-shot 2 g intravenous injection and 1 g intravenous drip infusion (1 hour) of AZT were very similar to the results obtained with the one-shot 1 g intravenous injection. (2) The residual serum concentration in neonates after one-shot 1 g intravenous injection of AZT to the mother was almost below the detectable limit. Transfer of AZT into milk was scarcely recognized. 2. Clinical studies (1) AZT was injected to 47 cases with various perinatal infections and it was more than "effective" in 45 cases with an efficacy rate of 95.7%. Also, all the 12 cases to which AZT was administered for prophylaxis of infections showed prophylactic effect. Bacterial eradication was obtained with 25 strains out of 29 aerobic Gram-negative bacteria, but 1 strain "persisted" and for 3 strains results were "unknown", hence an eradication rate of 96.2% was obtained. However, AZT treatment resulted in a little lower eradication rate against Gram-positive bacteria. (2) One case (1.3%) of minor degree of urticaria was found as a side effect, and one case each of eosinophilia and elevation of GOT, GPT and Al-P was observed as abnormal laboratory value. From the above results of pharmacokinetics and clinical evaluation, it has been concluded that AZT is a useful and highly safe drug in various perinatal infections and prophylaxis.