Effects of cefodizime (CDZM), a new injectable cephem antibiotic, on the intestinal bacterial flora were studied in tetra-contaminated mice and in pediatric patients. CDZM was intramuscularly administered at a dose of 100 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. For 3 species except E. faecalis, bacterial populations in feces were markedly reduced after the start of the treatment. Subjects in the pediatric study were 5 children with bacterial infections (4 boys and 1 girl) at ages from 7 months to 9 years 6 months and with their body weights ranging from 7.6 kg to 51.1 kg. CDZM was intravenously administered at a dose of 9.7 mg/kg to 23.0 mg/kg 4 times a day for 5 to 15 days. Although some variations in the fecal bacterial flora were noticed among these subjects during the treatment, populations of main aerobes and anaerobes such as Enterobacteriaceae, Enterococcus, Bacteroides, Bifidobacterium and Eubacterium decreased markedly in most cases. Glucose non-fermenting Gram-negative rods and fungi tended to increase during or after the administration of CDZM, and they were the most predominant species in some cases. Although these changes tended to return to predosing states after the cessation of the treatment with CDZM, attention must be paid to possible occurrences of diarrhea, superinfection or bleeding tendency when treatment with the drug is continued for long periods of time. Fecal concentrations of CDZM considered to be closely related to the changes of the intestinal bacterial flora showed pretty high values in all cases.