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Background: Overexpression of MDR (Multidrug Resistance) ABC transporters is a biomarker of multidrug resistance in hematological malignancies and other tumors.The P-glycoprotein/ABCB 1/MDR1-mediated multidrug resistance is probably still is the most widely observed mechanism in clinical multidrug resistance.The other ABC transporters implicated in multidrug resistance are (i) the multidrug resistance associated protein 1 (ABCC 1/MRP1), and (ii) the breast carncer resistance protein (ABCG2/BCRP/MXR).The SOLVO MDQ-kit has been approved for in vitro diagnostic use in the EU, folllowing laboratory and clinical validations.Therefore, testing of MDR-ABC transporter activity of clinical specimen is feasible.Aim: The goal of our study was to characterize interaction of cytostatic drugs, used in chemotherapy of malignant diseases with MDR-ABC transporters.Methods: Drug-transporter interactions were characterized using various in vitro systems that overexpress MDR transporters.The cytotoxicity assay is a widely used method to screen for resistance to cytotoxic drugs.Cell lines selectively overexpressing ABCB 1/ABCC 1/ABCG2 protein show resistance to drugs that are substrates of the given transporter.Calcein and Hoechst assays were used to confirm interaction between the compound and the selected transporter.Molecules that are substrates or inhibitors of the transporter increase cellular fluorescence by inhibiting the ABC transporter mediated efflux of the dye.Results: Over 25 drugs (e.g.alkylating agents, nucleoside analogues, antimetabolites, etc.) were tested.Overexpression ofABCB 1/ABCC 1/ABCG2 transporters resulted in increased resistance to 65%/52%/35% of compounds respectively.The inhibition of profile of interacting drugs was tested and characterized in dye efflux assays.The correlation between the two assays was limited as the cytotoxicity assay is a substrate assay and the dye efflux assay is an inhibition assay.Conclusions: Our data confirm that in the ABC transporter field substrate and inhibition assays must be separately carried out.Passive permeability of the compound is a critical parameter.In general, high passive permeability compounds are good inhibitors.On the other hand, high passive permeability may overcome resistance by MDR transporters.Passive permeability is needed to be taken in consideration when correlating substrate and inhibition assays.The cytotoxicity data are the most relevant data to assess a potential role of MDR-ABC transporters in drug resistance.