The multidrug resistance 1 (MDR1) gene encodes of the transmembrane P-glycoprotein (Pgp), involved in the transfer of a wide range drugs from the cytoplasm to the intercellular fluid. It is also known that the weak polymorphism of the C3435T MDR1 is associated with a higher risk of hematological diseases and the development of drug resistance in CML. But the influence of MDR1 gene expression level or the carriage of its genetic polymorphisms on the Pgp function in blood cells in CML and MPN patients has not been studied before. Purpose. Evaluation of daunorubicin accumulation as Pgp activity indicator in blood leukocytes in patients with myeloid disorders and the relationship with the C3435T MDR1 polymorphism as well as a level of the mRNA MDR1 expression. Material and methods. The study included blood samples from 195 patients with chronic myeloproliferative diseases, three patients with hereditary erythrocytosis and 92 healthy volunteers. The accumulation of potential Pgp substrate daunorubicin into lymphocytes and neutrophils was measured on a flow cytofluorimeter BD FACS Canto II. Polymorphism C3435T (rs1045642) of the MDR1 gene was determined by electrophoretic detection in an agarose gel. The expression of the mRNA MDR1 was determined by allele-specific PCR. Results. There was a decrease in the levels of accumulation of daunorubicin in lymphocytes and neutrophils in patients with chronic myeloproliferative diseases and hereditary erythrocytosis in comparison with the control group. However, expression of the MDR1 mRNA gene in leukocytes was reduced. The effect of polymorphism C3435T MDR1 on the accumulation of daunorubicin in the patient’s blood cells was not detected. No significant association with MPN or CML has been found. Conclusions. The mechanisms of daunorubicin accumulation in blood leukocytes in patients with chronic myeloproliferative diseases do not depend on the activity of the Pgp carrier, but can be caused by other systems of transmembrane transport or by a more intensive metabolism of the cytostatic that reduces its fluorescent activity.