The partial composition of O-antigens in Escherichia known as the causative agents of escherichioses localized in the intestine and other organs has been studied. Escherichia of groups 01, 02 and 03 are characterized by different partial composition of their O-antigens. This composition can be expressed by the formulae: 01a, 1b; 01a, 1b, 1c; 01a, 1b, 1c, 1d; 01a, 1e; 02a, 2b, 2c; 02a, 2b, 2c, 2d, 2e; 02a, 2b, 2d, 2e, 2g; 02a, 2c, 2e; 06a, 6b; 06a, 6c. No pronounced correlation between the partial composition of O-antigens within the O-group and the presence of different K- or H-antigens in the strain has been established. The scheme of adsorption has been developed and anti-Escherichia factor O-sera have been obtained. The use of these sera allows one to differentiate Escherichia strains, isolated from patients, within the O-groups according to the partial composition of their antigens, which is of both diagnostic and epidemiological importance.