Cefotaxime activity was studied in pediatric practice on the intestinal bacteria flora of 10 children, by means of a differential quantitative method for aerobic and anaerobic faecal flora and was compared to that of 41 controls not receiving any antibiotics. Cefotaxime was given alone in 7 children, and in combination with gentamicin in 3. An effect on the intestinal bacterial flora was noted on E. coli, which disappeared in 4 cases and diminished considerably in 5. A slight increase in Streptococcus D was observed without excessive multiplication of the flora. The study showed no significant alteration for the other aerobic or anaerobic bacilli. There was no selection of resistant organisms. Cefotaxime is a new cephalosporin which does not seem to produce an increase in many resistant pathogens due to a break-down of the barrier effect observed on the bacterial flora of the gut in children.