Enteropathogenic Escherichia coli (EPEC) is an important aetiological agent of persistent infantile diarrhoea. EPEC pathogenicity is not mediated through known toxins and the role played by outer-membrane proteins (OMPs) in the initial adherence of the bacterium, intimate attachment to epithelial cells and ultimately in the effacement of the intestinal epithelium is being pursued vigorously. In this study of the different cellular fractions of the bacterium investigated, only the outer-membrane fraction was able to disrupt HEp-2 cells. The outer-membrane fraction was also found to be cytotoxic and caused actin accumulation around the periphery of the host cells. To understand the role of OMPs in pathogenesis, protein profiles of outer-membrane preparations of wild-type and attenuated mutants lacking either the EPEC adherence factor (EAF) mega-plasmid or EPEC attaching and effacing gene A (eaeA) coding for a 94-kDa OMP, intimin or EPEC secretory protein gene B (espB) coding for a 34-kDa translocated signal transducing protein were compared and correlated with their cytopathic effects. A 43-kDa protein seen along with intimin in the outer membrane of EPEC was identified as maltoporin, an E. coli outer-membrane porin normally expressed only in response to maltose in the growth medium. In the case of EPEC, not only was this regulation lost, but also the expression of maltoporin was found to be tightly coupled to the expression of the secretory virulence factor EspB. Maltoporin per se is not toxic, as evidenced by the treatment of HEp-2 cells with the outer-membrane preparation of E. coli DH5a grown in the presence of maltose and the significance of this pathogenic adaptation is not clear. However, when maltoporin and possibly other unidentified proteins were not present as a component of the outer-membrane preparation, as in the outer-membrane preparation of an espB-negative strain, cellular disruption as well as actin accumulation proceeded at a very slow rate even though the cytotoxic effects were comparable to those of the wild-type EPEC strains.