A plasmid-free human Escherichia coli strain EMO, was inoculated to human newborns within two hours of life. It became established in the feces at a high population level. Ampicillin and tetracycline resistant E. coli strains disappeared or decreased to subdominant level in the inoculated newborns whereas they remained at a high level in the noninoculated newborns. Strain EMO was also shown to exert a barrier effect against a porcine plasmid-bearing enterotoxigenic Ent+ K88- E. coli strain in gnotobiotic mice. Axenic piglets monoassociated with Ent+ K88- E. coli strain died within 6 days. Axenic piglets diassociated with both EMO and the Ent+ E. coli survived longer, though no significant difference was observed between the population level of the Ent+ E. coli in the mono- and diassociated piglet feces. These results suggest that a prior inoculation with a non-pathogenic E. coli strain may improve the spontaneous ecological protection of piglets against Ent+ E. coli strains.