Among 48 clinical strains of Klebsiella pneumoniae isolated in a university hospital in Northern Greece, 29 (60.4%) exhibited resistance to third generation cephalosporins (3GC) and aztreonam. Thirty-two (66.7%) of the isolates were found resistant to the combination of ampicillin/sulbactam and six (12.5%) exhibited resistance to all the above antibodies plus cefoxitin. Resistance to 3GC was related mostly with the presence of a beta-lactamase exhibiting pI 8.2 and substrate profile of an SHV-5 type enzyme and rarely (in two of the cefoxitin resistant strains) with the presence of plasmid-mediated class C cephalosporinases. Resistance to the ampicillin/sulbactam combination was associated with the presence of a beta-lactamase with pI 5.4, presumably representing a TEM-1 beta-lactamase. These findings record a diversity of beta-lactamases and explain, at least partly, the various beta-lactam resistance patterns observed in our K. pneumoniae sample.