The functional fluorescent dye efflux assays are used in the study of the multidrug resistance of the malignant cells to the cytotoxic drugs which is caused by the overexpression of P-glycoprotein (P-gp) or another membrane transport system--a multidrug resistance related protein (MRP). P-glycoprotein and a multidrug resistance related protein are involved in the efflux of cytotoxic drugs out of the cell and are responsible for the resistance. Fluorescent dye efflux mediated by these proteins could be evaluated by the flow cytometry. This test seems to be an optimal approach to study the multidrug resistance. With help of the specific inhibitors such as verapamil and cyclosporine A, the functional capacity of these proteins and the possibility to overcome the multiresistant phenotype can be revealed. The cell line K562 with transfected P-glycoprotein gene serves as a model system for the studying of the transport function with the use of the fluorescent substrates and P-glycoprotein inhibitors.