Experimental and clinical trials were carried out with ceftizoxime in pediatric infections. Results were as follows. 1. The antibacterial activity of ceftizoxime against clinically isolated organisms was determined. Ceftizoxime was more active than cefazolin, cefotiam and cefmetazole against K. pneumoniae, E. coli and H. influenzae. 2. The serum concentrations of ceftizoxime following intravenous injection of 10 mg/kg were 16.2, 10.1, 6.2, 1.8 micrograms/ml at 30, 60, 120, 240 minutes after injection, respectively. Ceftizoxime was excreted with the rate of 97.2% in the 6-hour urine after injection. 3. Twenty-one patients comprising 5 with urinary tract infection, 16 with respiratory tract infection were received ceftizoxime at the doses ranging from 32 to 109 mg/kg divided 3 or 4 times a day. The results were excellent in 8 and good in 13 patients. The rate of satisfactory clinical response was 100%. 4. Slight and transient elevation of GPT was observed in 1 patient. Other side effects were not observed in any of these 21 patients.