Contemporary therapeutic procedures used in the treatment of malignant diseases are based on the no-selective action of cytotoxic or cytostatic substances. The sensitivity of tumour cells to cytostatics can be, however, reduced by the action of several mechanisms among which expression of the membrane glycoprotein P-170 is the most important. The latter actively transfers the cytotoxic substance from the cell and is responsible for multiple drug resistance-MDR). Glycoprotein P-170 can be assessed on leukaemic cells immunochemically by flow cytometry. A very perspective procedure is assessment of the in vitro sensitivity of leukaemic cells to clinically used cytostatics.