Treatment of infections caused by OXA-48 carbapenemase producing multidrug-resistant isolates often necessitates combination therapy. In vitro effect of different antibiotic combinations against multidrugresistant (MDR) Klebsiella pneumoniae isolates were evaluated in this study. Meropenem-tobramycin (MERtTOB), meropenem-ciprofloxacin (MERtCIP), colistin-meropenem (COLtMER), colistin-ciprofloxacin (COLtCIP) and colistin-tobramycin (COLtTOB) combinations were tested by time kill-assays. Each antibiotic alone and in combination at their Cmax values were tested against 4 clinical K. pneumoniae isolates at 1, 2, 4, 6, 8, 12 and 24 h. Effect of colistin and its associations were also assessed at 30 min. Bactericidal activity was defined as .3log10 CFU mL1 decrease compared with initial inoculum. Synergy was defined as .2log10CFU mL1 decrease by the combination compared with the most active single agent. Presence of blaOXA-48, blaNDM, blaVIM, blaIMP, blaKPC and blaCTX-M-1 genes was screened by PCR using specific primers. The blaOXA-48 gene was identified together with blaCTXM-1 group gene in all isolates. COLtMER demonstrated to be synergistic and bactericidal. MERtTOB showed synergistic and bactericidal effect on two strains although, regrowth was seen on other two strains at 24 h. MERtCIP exhibited indifferent effect on the strains. Combination therapy could be a potential alternative to treat MDR K. pneumoniae infections. This combination might prevent resistance development and secondary effects of colistin monotherapy. MERtTOB and MERtCIP might have an isolate-dependent effect, that may not always result in synergism.  Kulcsszavak: OXA-48 carbapenemase, MDR Klebsiella pneumoniae, time kill-assay, combination therapy