Background: Ineffectiveness of anticancer drugs is frequently observed in cancer chemotherapy. The resistance of tumor cells to various cytotoxic drugs is defined as multidrug resistance (MDR). The purpose of this study is to investigate the potential reversal effect of some synthetic and natural chemicals on drug-resistant MCF-7 cell lines. The effects of potential MDR modulators combined with some anticancer drugs were also studied. Methods: Flow cytometry, MTT cytotoxicity assays and checkerboard combination assays were performed to study the reversal of drug resistance and to investigate the antiproliferative effects of the combination of anticancer drugs and the potential modulators. The results indicated that verapamil, capsanthin, zeaxanthin and promethazine inhibited P-gp effectively, but chrysin was not effective at reversing the resistance in MCF-7 sublines. Four selective anticancer drugs ( paclitaxel, docetaxel, doxorubicin and vincristine) and 4 effective MDR modulators ( verapamil, capsanthin, zeaxanthin and promethazine) were applied to the sublines in combination. Results and Conclusion: Fractional inhibitory indices show that verapamil and zeaxanthin seem to be the most effective MDR reversal agents that may be used together with paclitaxel, docetaxel, vincristine and doxorubicin in drug-resistant mammary carcinoma sublines. In conclusion, this report represents the importance to find out active and efficient drug resistance modulators for improving the efficacy of chemotherapy. Copyright (C) 2008 S. Karger AG, Basel.