Objective:To observe the changes of resistance to daunorubicin(DNR) and the expression of multidrug resistance associated protein-1(MRP1) in acute monocytic leukemia cells THP-1 treated by cyclosporin A(CsA), and explore CsA’s function on chemotherapy sensitization to THP-1 cells as well as the related mechanism. Methods:IC50 of DNR to THP-1 cells, content and distribution of DNR in THP-1 cells treated by CsA were examined by MTT method and confocal laser scanning microscopy method, respectively. Expression amount of MRP1 were detected by immunocytochemistry and Western blot. Results:After treated with CsA, IC50 decreased obviously(1.536 4 ± 0.175 1 vs 0.558 8 ± 0.054 7,P < 0.01), content of DNR increasing obviously(21.40 ± 1.71 vs 131.96 ± 16.45,P < 0.01), DNR distributing uniformly,and expression of MRP1 increased(0.266 5 ± 0.004 2 vs 0.316 9 ± 0.006 2,P < 0.01). Conclusion:CsA can increase the sensibility of THP-1 cells to DNR, not through decreasing the expression of MRP1 in THP-1 cells. This might show that the resistance of THP-1 cells to DNR can’t be reduced by inhibiting the activity of MRP.