Two novel plasmid-mediated beta-lactamases (CTX-M-5 and CTX-M-6) produced by Salmonella typhimurium clinical strains were characterized. The enzymes exhibited a pI of 8.4, hydrolyzed oxyimino-beta-lactams and were susceptible to mechanism-based p-lactamase inhibitors. The respective bin genes were cloned and sequenced. The deduced amino acid sequences showed a high degree of homology with those of the previously described plasmid class A CTX-M-type enzymes and appeared related to the chromosomal p-lactamases of Klebsiella oxytoca. (C) 1998 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.