In a UK survey of the occurrence of extended spectrum beta-lactamases, 96 hospitals submitted a total of 3951 non-selected, non-duplicate isolates of Enterobacteriaceae from 100 patients in each hospital, 206 of these cultures being mixed and, therefore, discarded. These isolates were initially screened for strains likely to produce extended-spectrum beta-lactamases (ESBLs) by MIC determination of beta-lactams followed by a bioassay, then disc approximation test and isoelectric focusing (IEF). Isolates were further examined using two pairs of PCR primers for both blaTEM and blaSHV genes. The ability of isolates to transfer resistance to both cefotaxime and ceftazidime by conjugation and transformation were examined. Four hundred and nine cefotaxime/ceftazidime-resistant isolates (10.9%) were identified from the 3745 submitted isolates, of which 338 (9.0%) were Enterobacteriaceae, 29 Escherichia coli, 35 Klebsiella spp. and seven Hafnia alveii. IEF suggested that 17 isolates produced an ESBL, which was confirmed in most cases by PCR and hydrolysis, five isolates produced an SHV enzyme by IEF, but not confirmed by PCR, and 11 had isoelectric points in the range 8-9 suggesting a possible AmpC enzyme. Only two isolates transferred the determinants. In the case of the Klebsiella spp., 19 of the 24 ceftazidime-resistant/clavulanate-sensitive isolates were positive by PCR for a blaSHV gene. No isolates were identified as carrying blaTEM, although eight isolates had isoelectric points of 5-6.3, suggesting the presence of a possible TEM beta-lactamase. The results for the H. alveii isolates suggest that either an AmpC-like enzyme or a transferable beta-lactamase which is not TEM/SHV is present. This study shows that a wide range of genotypically and phenotypically different isolates of Enterobacteriaceae producing ESBL-like enzymes is present throughout the UK at a frequency of about 1% of unselected isolates. It is important that surveillance of resistance to these clinically important antibiotics is maintained as the occurrence of localized or more widespread outbreaks caused by bacteria producing ESBLs is to be expected.