Four human Lactobacillus acidophilus strains were tested for their ability to adhere onto human enterocyte like Caco-2 cells in culture. The LA 1 strain exhibited a high calcium independent adhesive property. This adhesion onto Caco-2 cells required a proteinaceous adhesion promoting factor, which was present in the spent bacterial broth culture supernatant. LA 1 strain also strongly bound to the mucus secreted by the homogeneous cultured human goblet cell line HT29-MTX. The inhibitory effect of LA 1 organisms against Caco-2 cell adhesion and cell invasion by a large variety of diarrhoeagenic bacteria was investigated. As a result, the following dose dependent inhibitions were obtained: (a) against the cell association of enterotoxigenic, diffusely adhering and enteropathogenic Escherichia coli, and Salmonella typhimurium; (b) against the cell invasion by enteropathogenic Escherichia coli, Yersinia pseudotuberculosis, and Salmonella typhimurium. Incubations of L acidophilus LA 1 before and together with enterovirulent E coli were more effective than incubation after infection by E coli.