A total of 160 Klebsiella pneumoniae clinical strains consecutively isolated in 14 Greek hospitals in a three-month period was examined. Application of capsular typing using 72 monovalent antisera combined with phage-typing using a set of 15 Klebsiella-specific phages showed the absence of epidemic strains. However, 41% of the isolates examined displayed high level resistance to ceftazidime and aztreonam and, in most of the cases, to more than one aminoglycoside as well as to other antibacterial drugs. Nearly all of these multi-resistant strains were epidemiologically distinct on the basis of their capsular serotype and phage reactivity. After examination of 14 distinct strains, it was found that in nine cases, the resistance characters were readily transferred to Escherichia coli recipients through large self-transmissible plasmids (15-100 MDa). Six of the nine plasmids had equal molecular weight (60 MDa) and displayed similar fragment profiles upon digestion with restriction endonuclease EcoRI. Isoelectric focusing and hydrolytic studies showed that the prominent beta-lactamase produced by the transconjugants harbouring the 60 MDa plasmids and the respective K. pneumoniae parent strains, was an extended-spectrum beta-lactamase of the SHV-5 type. It appears that among K. pneumoniae strains isolated in Greek hospitals a significant resistance rate to both newer beta-lactams and amino-glycosides has been established through the acquisition of promiscuous multi-resistant plasmids which share a high degree of similarity.