In this study, we investigated the antiproliferative activity of the isoflavones daidzein and genistein in three breast cancer cell lines with different patterns of estrogen receptor (ER) and c‑erbB‑2 protein expression (ERα‑positive MCF‑7 cells, c‑erbB‑2‑positive SK‑BR‑3 cells and ERα/c‑erbB‑2‑positive ZR‑75‑1). After treatment at various concentrations (1‑200 µM for 72 h), the effect of daidzein and genistein on the proliferation of different cell types varied; these effects were found to be associated with ERα and c‑erbB‑2 expression. Daidzein and genistein exhibited biphasic effects (stimulatory or inhibitory) on proliferation and ERα expression in MCF‑7 cells. Although 1 µM daidzein significantly stimulated cell growth, ERα expression was unaffected. However, genistein showed marked increases in proliferation and ERα expression after exposure to <10 µM genistein. Notably, the inhibition of cell proliferation by 200 µM genistein was greater compared to that by daidzein at the same concentration. Daidzein and genistein significantly inhibited proliferation of SK‑BR‑3 and ZR‑75‑1 cells in a dose‑dependent manner. In addition, ERα and c‑erbB‑2 expression was reduced by daidzein and genistein in both SK‑BR‑3 and ZR‑75‑1 cells in a dose‑dependent manner. However, the effect of genistein was greater compared to that of daidzein. In conclusion, the isoflavones daidzein and genistein showed anti‑breast cancer activity, which was associated with expression of the ERα and c‑erbB‑2 receptors.