Objective To explore the mechanism of the reversal of breast cancer resistant protein-mediated multidrug resistance by 17βestradiol. Methods Two BCRP expressing cell lines of MCF-7/MX20 and MCF-7/BCRP were established in which breast canrcer resistant protein(BCRP) was promoted by BCRP promoter and cytomegalovirus(CMV) promoter respectively.These drug resistant cell lines were cultured in medium containing 17βestradiol. Fourtyeight hours later,cytotoxicity assay,mitoxantrone efflux assays,quantitative RT-PCR were performed to observe the reversal function of BCRP by 17βestradiol on MCF-7/MX20 and MCF-7/BCRP respectively. Results After being treated with 17βestradiol,the intensity of mitoxantrone in MCF-7/BCRP was weaker than that in MCF-7/MX20 and the BCRP mRNA level in MCF-7/BCRP was high than that in MCF-7/MX20.The results of these experiments revealed that 17βestradiol could reverse the BCRP mediated multidrug resistance(MDR) in MCF-7/M