CTX-M and AmpC genes in human isolates of Escherichia coli, their genetic environment and their host plasmids were examined. Isolates (n=103) were selected based on resistance (minimum inhibitory concentration (MIC)> or =1 microg/mL) to ceftriaxone and cefotaxime. Polymerase chain reaction (PCR) and sequencing identified 29 isolates containing bla(CTX-M-15), 1 each of bla(CTX-M-2) (a strain originating from Israel) and bla(CTX-M-40), 20 isolates containing bla(CMY-7), 4 bla(CMY-2) and 1 bla(CMY-21). This is the first study of plasmid-mediated AmpC genes in E. coli in the UK. Eleven cefoxitin-resistant, AmpC PCR-negative isolates had ampC promoter region mutations. All bla(CTX-M-15) and 24 of 25 bla(CMY) genes were associated with an ISEcp1-like element. The bla(CTX-M-2) was located in an orf513-bearing class 1 integron. Plasmid restriction digests suggest transfer of genes between different plasmid backbones.