Experiments were focused on diarrhea prevention in weaned piglets caused by enterotoxigenic strains of Escherichia coli (ETEC) with colonizing factor 8813. An immunization procedure consisted of intramuscular application of ETEC strain bacterin a day before weaning and a peroral administration of a live culture of nontoxic E. coli strain with the same colonizing factor on the day of weaning. In an experiment on the litter of 10 piglets (six were immunized, four were controls), their intestines were colonized by the nontoxic E. coli strain for 4-7 days (Fig. 1). The challenge peroral infection by virulent ETEC strain demonstrated the protection of immunized piglets from the disease as well as from intestinal colonization by the administered ETEC strain. The same immunization procedure was tested on three pig farms with enzootic occurrence of diarrheas in weaned piglets. On these farms, besides ETEC strain with colonizing factor 8813 (F18) ETEC strains with other colonizing factors (K88, F not specified) were found out in the weanlings - Tab. I. Immunization effect was evaluated according to the rate of mortality of immunized and nonimmunized piglets within a fortnight after weaning. Out of 222 immunized piglets on S farm (Tab. II), 25 piglets died (11.3%), out of 232 nonimmunized animals it was 39 that died (16.8%). As for T farm (Tab. III), 22 piglets (8.6%) died out of 255 immunized animals while 71 out of control 274 piglets died (25.7%). A total of 3,692 were immunized on V farm (Tab. IV). Ninety-four animals died among them (2.5%). Mortality rate in the control group of 6,301 animals was 523 piglets (8.3%).