The inducible beta-lactamase of the clinical Proteus vulgaris isolate 4917/81 was highly purified by column chromatography and by FPLC (cation ion exchange column). Molecular weight of the enzyme amounted 33,000 daltons, as revealed by SDS-electrophoresis. The enzyme was not inhibited by p-chloromercuribenzoate, but by low concentrations of oxacillin and clavulanic acid. The enzyme inactivated not only penicillin derivatives (including ureidopenicillins), but also first-generation cephalosporins and above all oxime-cephalosporins such as cefuroxime, cefotaxime and related derivatives. Turnover rates of these agents were mainly influenced by the nature of substitution in 3' position of the cephalosporin nucleus. Breakdown was not detectable in compounds which were substituted in 6 alpha or 7 alpha position, respectively. The enzyme proved to be very sensitive to the nature of 6 alpha or 7 alpha substituent, as revealed by the study of enzyme kinetics; no turnover could be detected for the penem Sch29 482, imipenem, latamoxef, and aztreonam.