The effect of selected autoaggregative lactobacilli on enterohemorragic O157 Escherichia coli was studied. A total of 27 Lactobacillus sp. strains were examined for expression of autoaggregation and cell surface hydrophobicity. Autoaggregative isolates were obtained from the crops of chickens and vaginas of calves. Aggregative activity was seen between eight autoaggregative lactobacilli and six strains of enterohemorrhagic E. coli O157. The presence of aggregation-promoting factor (apf gene) was confirmed by polymerase chain reaction (PCR). We propose that autoaggregative lactobacilli that aggregate with enterohemorrhagic E. coli can express a class of APF proteins that exhibit the function of an aggregative mediator. The results of this study suggest that APF-producing lactobacilli could represent a further mechanism in the interaction of comensal microflora with enterohemorrhagic E. coli.