BACKGROUND: The molecular mechanisms of genistein's proliferative effects on breast cancer cells are largely unknown. This study aimed to examine estrogen-receptor (ER)-related signaling molecules involved in genistein-associated cell proliferation and survival (ERK1/2, p90RSK, JNK, Akt and NFκB) and to correlate these results to cell proliferation. MATERIALS AND METHODS: The effect of genistein on cell-signaling molecules was determined in T47D breast cancer cells by a Bioplex phosphoprotein detection kit. These results were confirmed by Western blotting and were correlated to cell proliferation by MTT assay. RESULTS: Low and high concentrations of genistein induced an ERK1/2-independent decrease in phosphorylated p90RSK. This effect was accompanied by decreased cell proliferation at high concentrations and an increased response at low concentrations of genistein following a 48-hour exposure. CONCLUSION: Concentration-dependent actions of genistein in T47D cells may be due to differential activation of signaling molecules.