In vitro experiments on 120 E. coli strains isolated from different sources (43 from water, 22 from the feces of healthy persons, 55 from patients with extraintestinal E. coli infections) have demonstrated that some of E. coli isolated, mainly the causative agents of extraintestinal infections, are resistant to the antibacterial action of "intercid" and have anti-Intercid activity (AIA) and a high level of antilysozyme activity. The use of the correlation and two-factor analyses has made it possible to establish the relationship between these characteristics and to find out an even more important contribution of AIA to the determination of bacterial Intercid-resistance (fusion force h2 = 13.1-25.0%). Experiments on E. coli isogenic variants with phenotypes AIA+ and AIA- have shown that AIA is an additional factor which determines the resistance of E. coli to "intercid".