The aim of this study was to evaluate the incidence of decreased susceptibility to broad-spectrum cephalosporins in Enterobacteriaceae that lack inducible chromosomal bla genes, and to determine the enzymes responsible for resistance. From all clinically relevant Enterobacteriaceae strains isolated between 1994 and 1996, 88 of 7054 Escherichia coli, seven of 581 Klebsiella pneumoniae and 23 of 166 Klebsiella oxytoca strains were studied because of their decreased susceptibilities to broad-spectrum cephalosporins (as reflected in intermediate susceptibilities and/or positive synergy tests and/or irregular crenellated inhibition zones). The most frequent mechanism implicated in decreased susceptibility to broad-spectrum cephalosporins displayed by E. coli and K. oxytoca was hyperproduction of chromosomal β-lactamase, followed by plasmid-mediated SHV-1 hyperproduction in E. coli. In our hospital, the incidence of plasmid-mediated extended-spectrum β-lactamases (ESBLs) between 1994 and 1996 was low. ESBLs were found in only 10 (0.14%) E. coli strains (six CTX-M-9, two TEM-12 and two SHV-2), in one (0.17%) K. pneumoniae strain (SHV-2) and in no K. oxytoca strains. The relatively wide variety of β-lactamases that were detected among these common bacteria isolated from a single medical centre, including non-TEM- and non-SHV-derived ESBLs, appears epidemiologically remarkable.