A β-lactamase produced by Pseudomonas stutzeri was purified to protein homogeneity, and its physicochemical and catalytic properties were determined. Its profile was unusual since, in addition to penicillins, the enzyme hydrolysed second- and third-generation 'β-lactamase-stable' cephalosporins and monobactams with similar efficiencies. On the basis of the characteristics of the interaction with β-iodopenicillanic acid, the enzyme could be classified as a class-A β-lactamase. However, when compared with most class-A β-lactamases, it exhibited significantly lower k(cat) /K(m) values for the compounds usually considered to be the best substrates of these enzymes.