Objective: To investigate the reversal effect of CQ11, a chloroquine derivative, on multidrug resistance (MDR) in doxorubicin (DOX)-resistant human breast carcinoma cell line MCF/DOX. Methods: Cells of a human breast cancer cell line, MCF, and its DOX-resistant variant, MCF/DOX, were cultivated with DOX and/or CQ11. The cytotoxicity of drugs in vitro was assayed by MTT method. The accumulation of DOX in these cells was detected by fluorescence spectrophotometer. Results: MCF/DOX cells were 119 times more resistant to DOX in comparison with MCF cells. After simultaneous treatment with CQ11 at the concentrations of 1.0, 2.5 and 5.0 μmol/L, the IC50 of DOX for MCF/DOX cells decreased from 3.1±0.47 μmol/L to 0.58 ±0.032, 0.19±0.012 and 0.081±0.015 μmol/L, respectively, thus, increasing the DOX sensitivity by 5.3-fold (P<0.01), 16- fold (P < 0.01) and 38-fold (P < 0.01), respectively. In the accumulation assay of DOX, simultaneous incubation of MCF/DOX cells with CQ11 significantly increased the DOX accumulation in MCF/DOX cells. No such results were found in parental MCF cells. Conclusion: CQ11 had strong MDR reversal effect by enhancing intracellular DOX accumulation in MCF/DOX cells, indicating that CQ11 may be a promising MDR chemosensitivity.