The influence of cefteram pivoxil (CFTM-PI), a new oral cephalosporin, on the intestinal bacterial flora was studied in tetra-contaminated mice and in pediatric patients. CFTM-PI in fine granular form was administered at a dose of 10 mg/kg once a day for 5 consecutive days to mice contaminated with 4 different species of organisms: Escherichia coli, Enterococcus faecalis, Bacteroides fragilis and Bifidobacterium breve. No remarkable change was observed in the fecal viable cell count except that slight decrease in E. coli count was observed on days 2 to 5 after starting administration. The subjects in pediatric study were 5 children, 3 boys and 2 girls at ages from 6 months to 10 years 4 months, with infections. Their body weights ranged from 3.5 to 28.0 kg. CFTM-PI in fine granular form was administered at each dose of 3.0 to 3.8 mg/kg, 3 times daily for 5 to 11 days. During the administration of CFTM-PI, there were some variations in the change of the fecal bacterial flora noticed between subjects. Although Enterobacteriaceae tended to decrease and Enterococci tended to increase, other main aerobes and anaerobes were almost unchanged in most cases. There was no case in which glucose non-fermenting Gram-negative rods and fungi became predominant. In a 10 years 4 months old boy, the fecal concentration of CFTM, the active form of CFTM-PI, was 72.20 micrograms/g and a remarkable decrease of total anaerobe count was observed. In feces, CFTM-PI and CFTM were detected in 4 and 2 cases respectively, and their concentrations were 1.50 approximately 89.65 micrograms/g and 2.25 approximately 72.20 micrograms/g, respectively. beta-Lactamase activities in feces were positive in all cases. From the above, CFTM-PI is considered to be a drug with relatively less influence on the intestinal bacterial flora. But as high concentrations of drugs were detected in feces under some circumstances, we need to pay attention to fecal drug concentrations.