To confirm the existence in nature of Salmonella enteritidis strains of different degrees of virulence and to elucidate the mechanisms underlying the effects of such strains on the epithelial barrier function, the consequences of infection of Caco-2 cells and HEp-2 cells with 15 S. enteritidis strains in a chicken infection model were examined. The more virulent strains of S. enteritidis, which are biofilm producers in adherence test medium, were able to disrupt HEp-2 and Caco-2 monolayers, as shown by transmonolayer electrical resistance and lactate dehydrogenase activity. In contrast, the low-virulence strains of S. enteritidis, which do not produce biofilms in adherence test medium, had no effect on the same cells. An avirulent rough mutant of Salmonella minnesota exhibited a pattern of behaviour similar to that of the low virulence strains of S. enteritidis, whilst a clinical Salmonella typhi strain caused rapid injury to the monolayers. The effect of supernatants of Salmonella cultures in adherence test medium on the integrity of Caco-2 cell monolayers indicated that the high-virulence S. enteritidis strains, but not the low-virulence strains, release a soluble factor when incubated under optimum biofilm-forming conditions, which enables the disruption of the integrity of Caco-2 monolayers.