The structural properties of EspB, a virulence factor of theEscherichia coli O157 type III secretion system, were characterized. Far-UV and near-UV CD spectra, recorded between pH and pH , show that the protein assumesa-helical structures and that some tyrosine tertiary contacts may exist. All tyrosine side-chains are exposed to water, as determined by acryl-amide fluorescence quenching spectroscopy.