Clinical pharmacology and safety of aztreonam (AZT) in the neonatal period were investigated. The results obtained are summarized as follows. 1. Serum concentrations of AZT at 30 minutes after administration of 10 mg/kg were 22.1-32.2 micrograms/ml and those of 20 mg/kg 22.5-75.9 micrograms/ml. 2. Serum half-lives of AZT were 3.5-6.6 hours in 0-3 day-old neonates, and 2.0-4.0 hours in neonates 4 day-old or older. 3. A dose response was evident between the 10 mg/kg administration group and the 20 mg/kg group. 4. Urinary recovery rates of AZT in the first 6 hours after administration ranged between 17.8 and 69.9%. 5. No clinical side effects were observed in the administration of AZT alone (6 cases), or in combination with ampicillin (9 cases). Thrombocytosis was observed in 1 case as an abnormal laboratory finding, but it returned to normal within 1 week after the completion of AZT administration. 6. AZT had a potent antimicrobial activity against Gram-negative aerobes and hardly induced beta-lactamase. Furthermore, side effects were not observed in this study. Therefore, AZT is considered to be useful for the treatment of urinary tract infections and other serious infections caused by Gram-negative pathogens even in the neonatal period.