Clinical usage of aztreonam (AZT), a newly synthesized antibiotic which belongs to monobactam, was evaluated for its efficacy and safety in 22 patients aged from 1 month-old to 13 year-5 month-old with bacterial infections and the following results were obtained. AZT was administered to 4 patients with pyelonephritis and 10 patients with tonsillitis at a daily dosage of 40.4-120.9 mg/kg and to 5 patients with clinical sepsis associated with agranulocytosis caused by intensive antileukemic therapy at a daily dosage of 142.4-171.4 mg/kg, divided into 3 or 4, by intravenous injection or by 30 minutes drip infusion. The clinical results of these 19 evaluable patients were as follows: excellent; 10 cases, good; 5 cases, fair; 2 cases, poor; 2 cases. The over all efficacy rate was 78.9% and that of pyelonephritis and tonsillitis was 100.0%. No clinical side effects were observed in any 23 patients, including a patient who proved to be suffering from Mycoplasma pneumoniae infection, and no abnormal laboratory findings caused by AZT was noticed. The MICs of AZT against 9 strains isolated from patients with pyelonephritis and those with tonsillitis were as follows: MICs against all of 3 strains of K. pneumoniae were less than 0.05 microgram/ml. MICs against 2 out of 4 strains of H. influenzae were less than 0.05 microgram/ml and those of the remaining 2 strains were 0.10 microgram/ml. MIC against 1 strain of S. aureus was 1.56 microgram/ml. MIC against 1 strain of S. epidermidis was more than 100 micrograms/ml.(ABSTRACT TRUNCATED AT 250 WORDS)