Longitudinal quantitative cultures of fecal flora of 20 newborns, 4 older babies and 10 healthy adults were carried out to study the composition and development of the intestinal flora. In all newborns the same sequence of colonization was observed. The numbers of aerobic and anaerobic bacteria fluctuated and reached finally numbers of 10(10)/g wet weight. In adults the flora was in balance with 10(5)-10(7) aerobic and 10(10)-10(11) anaerobic bacteria/g wet weight. Interaction experiments in vitro showed growth inhibition of Bacteroides fragilis by all intestinal species isolated. Bifidobacteria were not inhibited. The assumption was made that this type of interaction could be one of the mechanisms involved in the intestinal micro-ecology. Three of the Bacteroides fragilis strains tested were able to grow on "natural intestinal substrates" as gastric mucin, glycogen and a variety of plant polysaccharides. Acetic, lactic, propionic and succinic acids were detected as fermentation products.