Strains of three subgroups of Escherichia coli O128 were studied. Enterotoxin production was observed in 30 (91%) O128ac strains, whereas strains of subgroups O128ab and O128ad were not toxigenic. CFA/I was only found in two serotypes of subgroup O128ac, all of them producing heat-stable enterotoxin except for one which produced both toxins. None of the strains studied produced CFA/II. In a binding test with HeLa cells, localized adherence was found only in strains of subgroup O128ab; diffuse adherence occurred in strains of subgroup O128ac. As flagellar antigens were specific in subgroups ab and ac and toxin production was observed only in subgroup ac, the present results suggest that subgroup and serotype are useful markers for O128 strains that are enterotoxigenic or enteropathogenic.