OBJECTIVE The aim of the present study was to investigate the reversing effect of levistolide A (LA) on P-glycoprotein (P-gp)-mediated multidrug resistance (MDR) in human breast carcinoma Bcap37/MDR1 cells. METHODS After chemotherapeutic drugs (adriamycin or vincristine) used alone or in combination with LA, cell proliferation was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide assay and cell cycle distribution by flow cytometry. RT-PCR was used to detect MDR1 gene transcription and the Western blot assay was used to assess P-gp expression and the cleavages of poly(ADP-ribose) polymerase and caspase-3. Apoptosis was detected by terminal transferase-mediated dUTP nick end-labeling assay. Moreover, the P-gp function was evaluated by the intracellular accumulation of the P-gp substrate detected by flow cytometry. RESULTS We found the subcytotoxic doses of LA significantly enhanced adriamycin- or vincristine- induced G2/M arrest and apoptosis. These effects were consistent with the ability of LA to inhibit P-gp function. Moreover, LA dramatically enhanced the verapamil (VER) ability to reverse drug resistance. CONCLUSIONS LA has the potential to be developed as a novel P-gp modulator. Furthermore, the combination of LA and VER might represent a more sufficient but less toxic anti-MDR regimen.