The monobactam antibiotic aztreonam (AZT) was administered by intravenous drip infusion to women with intrauterine infections at 6-hour intervals and its concentrations in the maternal blood, umbilical blood, and umbilical tissue were determined at parturition to evaluate its transport to the umbilicus and fetus, and this accumulation in the tissue. Further, umbilical tissue was cultured to detect bacteria and examined histologically to evaluate the efficacy of AZT its treatment of omphalitis. Neither maternal nor umbilical blood showed any signs of drug accumulation, and there was a correlation between maternal and umbilical blood levels of AZT and time lapsed after most recent administration. Umbilical tissue levels of AZT tended to increase with time for about 12 hours after the start of administration, but showed no tendency to further rise thereafter. The umbilical tissue cultures were negative for causative pathogens 12 hours or more after the initial dosing. Histological examination of umbilical tissue was negative for omphalitis 12 hours or more after the start of administration in all but one patient. After the administration of AZT to pregnant women with intrauterine infections, it was detected in the umbilicus and fetus in high concentrations at parturition without appreciable tissue accumulation, newborn infection or adverse reaction. The overall results suggest that AZT is clinical effective when used in the treatment of omphalitis.