The basic and clinical efficacies of aztreonam (AZT) were evaluated in obstetric and gynecologic infections during the perinatal period. The results obtained are summarized below. 1. Concentrations of AZT in umbilical venous blood and amniotic fluid showed a good placental transfer of AZT. The rate of placental transfer of AZT was very similar to rates for cephalosporins. 2. Since the clinical efficacy rate of AZT in the treatment of Gram-negative bacterial infections was 80.0%, AZT appears to be a useful drug against Gram-negative bacterial infections.