A septicaemia Klebsiella pneumoniae isolate from Greece (Ath1) was shown to be resistant to third generation cephalosporins, aztreonam and cefoxitin and this resistance was not decreased in the presence of clavulanic acid. The gene coding for the resistance phenotype, associated with a beta-lactamase showing cephalosporinase activity and a pI of 8.6, could be transferred into Escherichia coli K-12 by conjugation and transformation. DNA-hybridisation showed that this gene was located on two different plasmids, 7.8 and 80 kb respectively. The larger, conjugative plasmid also carried genes coding for another beta-lactamase (pI 6.6) and resistance to aminoglycosides, tetracycline, chloramphenicol and trimethoprim. Under highly stringent conditions the gene coding for the pI 8.6 beta-lactamase hybridised with chromosomal DNA from Citrobacter freundii OS60 but not from E. coli or Enterobacter cloacae. Furthermore, the restriction map of this beta-lactamase gene corresponded to that of ampC from C. freundii OS60. The regulatory ampR gene could not be detected in the plasmid DNA from the Ath1 K. pneumoniae by DNA hybridisation. The relationship between a plasmid-mediated extended spectrum beta-lactamase in a K. pneumoniae septicaemia isolate and chromosomal AmpC beta-lactamase from C. freundii was demonstrated.