We isolated a clinical Escherichia coli strain with an antimicrobial resistance phenotype characteristic for the expression of an AmpC beta-lactamase. Molecular methods revealed a novel, plasmid-localized variant of CMY-2 with a substitution of valine 231 for serine (V231S), which was designated CMY-42. Like the CMY-2-like AmpC beta-lactamase CMY-30, carrying the substitution V231G, CMY-42 displayed increased activity toward expanded spectrum cephalosporins. This finding supports the hypothesis that a bulky side chain at position 231 (Ambler's position 211) may pose a steric clash with certain cephalosporins hindering the access of the AmpC beta-lactamase; however, additional phenomena may account for the observed hydrolytic properties.