Serum concentration, urinary excretion and clinical application of aztreonam (AZT) were studied as follows: 1. Serum concentrations of AZT 1 hour after intravenous injection were 21.0 micrograms/ml in 1 case administered with approximately 10 mg/kg drug and 44.2 micrograms/ml on the average for 7 cases given approximately 20 mg/kg, indicating that serum concentrations are dose-dependent. Average serum half-life in 3 mature babies was 4.75 hours and that in 4 premature babies was 6.59 hours thus T 1/2 was longer in the latter. T 1/2 of 64 days of age newborn was 3.80 hours. Urinary recovery rates in 2 cases examined were 52.1 and 51.9%. 2. Daily dosages of AZT 39.9-63.3 mg/kg were intravenously administered to 10 newborns and prematures b.i.d. or t.i.d., 5 cases of which received AZT alone and the other 5 received AZT in combination with ampicillin (ABPC). Of the above 10 cases, AZT was given to 8 cases for treatment and to the other 2 cases for prophylaxis. Excluding 2 unascertainable cases, AZT showed good or better effectiveness in all the 6 cases in the treatment group, i.e., sepsis 1, suspected sepsis 1 and urinary tract infection 4 cases. All the identified pathogens (Escherichia coli 2 strains, Klebsiella pneumoniae 1 strain and Enterobacter 2 strains) were eliminated by the treatment. No onset of infection was observed in either of the 2 cases with prophylaxis. One of them was administered with AZT for 52 days consecutively but neither side effect nor abnormal laboratory test value was observed. 3. Side effect was not observed at all. One case each of minor degree of platelet increase and GOT elevation was recorded as an abnormal test value. The elevated GOT value continued to be high even after the completion of the administration and it was presumed to be due to the primary disease, heart failure. 4. As results of the above studies, AZT was considered to be effective and safe for neonatal infections caused by Gram-negative bacteria. It may be safer to initiate the treatment with AZT and ABPC in combination than with AZT alone before the identification of pathogen and to change the therapy to single administration of either AZT or ABPC when the pathogens are identified. With respect to method of administration, AZT 20 mg/kg 2 or 3 times a day appeared to show expected efficacy for the newborns with in 7 days after birth.