The content of enterobacteria in fecal samples obtained from 1,146 patients with acute enteric infections was studied. In the first days of the disease about 70% of the patients were found to have abundant and highly diversified intestinal microflora with Escherichia contamination of feces constituting 10(6)-10(7) microbial cells/g in children and 10(7)-10(8) microbial cells/g in adults. The number of patients excreting Escherichia and pathogenic enterobacteria increased from 8.8% (children under 6 months) to 28.6% (adults), the number of patients excreting Escherichia and opportunistic enterobacteria decreased from 69.1% (children under 6 months) to 36.8%. In 75% of cases the reverse relationship between the number of Escherichia and opportunistic enterobacteria was registered, the latter numbering two and more genera in 50% of cases. The serological picture of Escherichia was poorer when they were isolated simultaneously with shigellae (83 O-groups) than in those cases when Escherichia alone were isolated (108 O-groups), or when Escherichia were isolated simultaneously with opportunistic enterobacteria (119 O-groups).