Aztreonam (AZT) was studied for its pharmacokinetics, clinical effect and effect on intestinal bacterial flora in neonates, and the results obtained are summarized as follows: 1. Serum concentrations of AZT upon intravenous administration of 20 mg/kg were 38.6 micrograms/ml in 30 minutes, 30.6 micrograms/ml in 1 hour and 13.6 micrograms/ml in 6 hours. The T 1/2 was 3.73 hours. Ampicillin (ABPC) 25 mg/kg was concurrently used with AZT in 2 cases and serum concentrations of AZT in these 2 cases were 40.3 and 36.9 micrograms/ml in 30 minutes, 35.7 and 32.6 micrograms/ml in 1 hour, and 13.1 and 10.2 micrograms/ml in 6 hours, respectively. T 1/2's were 3.32 and 2.91 hours, respectively, and no interaction between the 2 drugs was observed. 2. AZT was administered to 21 neonates between 0 and 83 days of age and ABPC was concurrently administered to 18 of the cases. Clinical evaluation was made in 14 cases, where AZT was remarkably effective in 7 cases, effective in 6 cases and not effective in 1 case. Of the 21 cases, 1 case of diarrhea, 1 case each of eosinophilia, an increase in platelets, an increase in platelets and an elevation of GOT and a decrease in platelets were recorded. 3. With respect to effects of AZT on intestinal bacterial flora, fecal concentrations of AZT upon its single administration to 2 cases were low suggesting, there was little effect on the intestinal flora. Some effect on anaerobes, however, was recognized in 4 cases in which ABPC was concurrently used.