The adherence of Escherichia coli of certain adhesin types can be influenced by subinhibitory concentrations of β-lactam antibiotics. Subinhibitory concentrations (¼MIC) of those penicillin and cephalosporin compounds which lead to filament formation increase mannose-sensitive adhesion (MS+). Antibiotics with ovoid cell formation did not change or only slightly reduced MS+ adherence. Strains with mannose-re-sistant (MR+) adhesins were not affected by these antibiotics. Only azthreonam increased the adherence of both MS+ and MR+ strains. Vancomycin and fosfomycin reduced the adherence of all strains tested. The degree of change was strain and antibiotic specific. No relation could be detected between the changes of adherence and the changes of hydrophobicity under the influence of antibiotics. Electron microscopic investigation has not yet yielded an explanation of the observed phenomenon.