Thirty-seven strains of Escherichia coli O26:H11 from infants and calves with diarrhea were examined for properties associated with enteropathogenic (EPEC) or enterohemorrhagic E. coli (EHEC). Strains were heterogeneous with respect to Vero cytotoxin (VT) production and hybridization with the EHEC plasmid-specific (CVD419) probe; 26 strains produced VT1; 1 produced VT2. Twenty-four of 27 VT+ strains and 5 of 10 VT- strains hybridized with the CVD419 probe and produced enterohemolysin; these properties are characteristic of EHEC. The strains did not hybridize with the EPEC adherence factor probe, a property characteristic of some EPEC. Nevertheless, 36 strains adhered to HEp-2 cells in a localized manner and were positive by the fluorescence actin staining (FAS) test that is considered to correlate with the ability to cause attaching and effacing lesions in vivo. EPEC and EHEC cause these lesions. Although the FAS test appeared to be the most general pathogenicity test for the O26:H11 strains, it could not be used to assign strains specifically to EPEC or EHEC groups.