A differential quantitative analysis was used to study the effect of cefotaxime on the fecal flora in 26 hospitalized children ranging from two days to four years of age. Fecal specimens were obtained before, during and after therapy. This study was evaluated in comparison to 41 patients of the same age and from the same environment without antibiotic treatment or signs of infection. The fecal flora of the control group showed qualitative and quantitative stability. Two groups of species were distinguished: a group in which the upper limit was less than or equal to 10(7) bacteria/g of stool (Klebsiella, Enterobacter, other enterobacteria, Staphylococcus, Pseudomonas) and a group with less than or equal to 10(10) bacteria/g of stool (anaerobes, Escherichia coli, Streptococcus D). This stability of the control group of subjects allowed us to interpret the variations of microbial concentrations during and after cefotaxime treatment. In previous studies, we showed in the newborn a substantial risk of septicemia of intestinal origin when overgrowth occurred, especially with Klebsiella. With cefotaxime there was a decrease or a disappearance in 65% of E. coli and a slight decrease of Klebsiella and Enterobacter. This fact was of great interest in the treatment of endogenous secondary septicemia. We observed an appearance of Pseudomonas (nine Pseudomonas aeruginosa, two Pseudomonas putida, one Pseudomonas fluorescens) in 12 cases among the 26 children treated.(ABSTRACT TRUNCATED AT 250 WORDS)