Fundamental and clinical studies were carried out with aztreonam (AZT), a new monocyclic beta-lactam antibiotic, in pediatric infections. Results were as follows. The mean half-lives in the vein blood were 1.09 hours, 1.18 hours, 1.22 hours after injection, when the doses were 10, 20 and 40 mg/kg, respectively. Dose response was observed. The average recovery rates in the urine between 0 and 6 hours were 40.2%, 42.3%, 50.8% when the doses were 10, 20 and 40 mg/kg, respectively. The antibacterial activity of AZT against 16 clinical isolates were determined in comparison with those of ABPC, CPZ, LMOX and CTX. Against 8 clinical isolates of E. coli and 3 of H. influenzae, the activity of AZT was equal or superior to that of CPZ, LMOX and CTX, and way by far superior to that of ABPC. Twenty-three pediatric patients received AZT in doses ranging from 48 to 79 mg/kg divided 3 times a day; 12 cases of urinary tract infection, 9 cases of respiratory tract infection and 2 cases of bacterial enterocolitis. The rate of clinical effectiveness was 100%. No side effect was observed. Slight elevation of GOT and GPT were observed in 2 cases, increase of platelet count in 2. All were considered to be transient and mild.