Fundamental and clinical studies were carried out on ceftizoxime suppository (CZX-S), and the following results were obtained in pediatrics. In 4 patients of the CZX-S 125 mg-administered group (9.4-9.9 mg/kg), the serum concentration of CZX reached a peak of 5.55 micrograms/ml on the average, 30 minutes after dosing, i.e. at the time of initial blood collection, and decreased gradually to 0.20 microgram/ml 6 hours after dosing. The half-life was 1.09 hours. In 5 patients of the CZX-S 250 mg-administered group (8.4-18.1 mg/kg), the serum concentration of CZX peaked at 7.07 micrograms/ml on the average and then gradually declined to 0.16 microgram/ml 6 hours after dosing. The half-life was 1.00 hour. The urinary recovery rate varied as widely as 6.5-38.0% in all the patients of both groups. CZX-S was given to total 19 patients; 8 patients with urinary tract infection (UTI), 3 with pharyngitis or tonsillitis, 4 with bronchitis, 2 with pneumonia, 1 with otitis media and 1 with staphylococcal scalding skin syndrome. The overall effect of CZX-S in 15 patients was "effective" or better response, with an effectiveness rate of 83.3%, except one who discontinued the drug because of side effects. CZX-S was given to most of the patients weighing 15 kg or higher in a dose of 250 mg 3-4 times a day and frequently to patients weighing less than 15 kg in a dose of 125 mg 3-4 times a day. As to side effects, slight diarrhea was encountered in 1 patient. Laboratory examinations disclosed an increase in GOT in 1 patient, which returned to normal after continual insertion of the suppository.