Influences of antibiotics on the fecal flora in children were studied for oral ampicillin, penicillin V, erythromycin, cefaclor, and gentamicin and for intravenous ampicillin, methicillin, cefpiramide, and ceftazidime. All antibiotics affected the normal flora, although the quality and quantity of the changes were variable. No substantial differences were noted between the oral and intravenous use of ampicillin with regard to its effect on the flora. Three penicillins, ampicillin, penicillin V, and methicillin, caused remarkable changes. The characteristic pattern observed was the considerable suppression of Bifidobacterium, Streptococcus, and Lactobacillus species. Although enterobacteria did not significantly change in number, Klebsiella spp. frequently replaced Escherichia coli. In patients given erythromycin and cefaclor, the reduction in the number of Bifidobacterium spp. was 1 log10 and that of members of the family Enterobacteriaceae was 3 log10. Gentamicin administered orally caused a drastic change, including a remarkable decline of E. coli to less than 2 log10/g of feces. Cefpiramide, a parenteral expanded-spectrum cephalosporin, suppressed normal flora so markedly that almost all species of organisms were eradicated, and the active growth of yeasts was promoted (2.6 log10 increase). Ceftazidime caused similar changes as cefpiramide, but the changes were less extensive. Yeasts increased after treatment with most antibiotic groups. This increase was particularly prominent in patients given oral penicillins and expanded-spectrum cephalosporins.