A protein-engineered beta-lactamase, constructed by site-directed mutagenesis in Escherichia coli (E104M/G238S), and having broadened specificity, was able to degrade cephalosporins of first, second, and third generations. Manipulations of culture conditions allowed an increase in beta-lactamase specific activity by up to twofold. The resultant bacteria were used to construct an immersable whole-cell biosensor for the detection of new-generation cephalosporins. Cells were immobilized on agar membranes, which in turn were attached to the surface of a flat pH electrode, thus constituting a biosensor based on the detection of pH changes. The sensor was able to detect second- and third-generation cephalosporins : cefamandole (0.4-4 mM) cefotaxime (0.4-3.5 mM), and cefoperazone (0.3-1.85 mM). Response times were between 3.5 and 11 min, depending on the kind of cephalosporin tested. The biosensor was stable for at least 7 d, time during which up to 100 tests were performed.