Cefpirome (CPR, HR810), a new parenteral cephalosporin antibiotic, was studied for its effect on the intestinal bacterial flora in pediatric patients. The subjects were children admitted for infections (6 males and 3 females, 1 month to 5 years 1 month old, weighted 3.94 to 21.0 kg). CPR was intravenously administered at a dose between 19.0 to 40.0 mg/kg, 3 to 4 doses daily over 6 to 12 days. The feces from these children were collected before, during, and after administration, and bacteria were identified and counted. CPR concentration, beta-lactamase activity, and Clostridium difficile D-1 antigen were also assayed. Bacterial flora changes in feces during CPR administration showed some variance, but generally 5 cases out of the 9 showed a significant decrease in Enterobacteriaceae and Enterococcus faecalis among aerobic bacteria. The other 4 cases showed some transient decrease, but no significant change was observed. No significant changes were recognized for Enterococcus avium and Enterococcus faecium, and the total aerobic bacterial count decreased in a transient manner in only one patient. Regarding anaerobic bacteria, Bifidobacterium and Eubactrium revealed a significant decrease, a transient decrease or no change from case to case. Bacteroides showed little change in count. Consequently, the total anaerobic bacteria count did not reveal a large change aside from 1 case in which Bacteroides was not detected before administration and a significant decrease of other bacteria was noted. In no case, glucose nonfermentative Gram-negative bacilli or fungi were found dominant. Although C. difficile and C. difficile D-1 antigen were detected in 3 and 4 cases, respectively, there was no exact relationship between the number of C. difficile and the characteristics of the feces. CPR was detected in fecal samples from 6 cases during administration with concentrations ranging between 1.20 to 22.4 micrograms/g. High values of CPR tended to be found in specimens with low beta-lactamase activity in the feces. When drug sensitivities of the bacteria isolated from feces before and after administration were compared, higher levels of resistance were found in some bacteria such as Enterococci and Bacteroides during or after administration than before administration. The above results suggest that CPR is a drug with a relatively small influence on the intestinal bacterial flora in children, but a particular attention is required for diarrhea and microbial replacement during a continuous, long-term administration of the drug.