OBJECTIVE: To investigate the anti-breast cancer(BC) effects and mechanisms of action of Xihuang pill(XHP) by conducting in vitro experiments on human BC cell lines. METHODS: Two human BC cell lines(MCF-7 and MDAMB231) were cultured and treated with XHP. Cell viability was detected using the 3-(4, 5-Dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide(MTT) assay. Flow cytometry was used to measure the cell cycle and apoptosis. The cell cycle was analyzed with propidium iodide staining. Apoptosis was evaluated using the Annexin V-fluorescein isothiocyanate/propidium iodide method. Western blotting was used to analyze the expression of estrogen receptor(ER)-α and ER-β.RESULTS: XHP had growth-inhibitory effects on MCF-7 and MDA-MB231 cells with a half-maximal inhibitory concentration(IC50) of 10.14 mg/mL(MCF-7) and 8.98 mg/mL(MDA-MB231). Apoptosis was induced to some extent. Certain changes in the ER were caused. Upregulation of ER-α protein was found in MCF-7 cells. ER-β expression in MDA-MB231 cells was increased. Cell-cycle arrest was not observed in the two BC cell lines. ER-β expression in MCF-7 cells was unchanged. No ER-α expression was shown in MDA-MB231 cells. CONCLUSION: These data suggest that XHP can affect cell viability and cause apoptosis, but that the cell cycle is not blocked. XHP has a certain impact on ER expression, but its mechanisms of action of anti-BC effects may not be due to regulation of ER expression.