The occurrence of positive synergy between antibiotic discs of amoxicillin/clavulanate and cefoperazone was registered in two Klebsiella pneumoniae strains, isolated from hospitals in Czech and Slovak Republic, indicating the presence of genes coding for an extended-spectrum beta-lactamase active also against cefoperazone, a broad-spectrum cephalosporin. Sulbactam inhibited the hydrolysis of cefoperazone by cell-free lysates of these strains which substantiates its use in combination with cefoperazone. Resistance to cephalothin, cefotaxime, ceftazidime, cefoperazone, cefepime and aztreonam was transferred from K. pneumoniae isolates to Escherichia coli K-12 3110 and to Proteus mirabilis P-38 recipient strains.