['To explore the effect and pathway of phytoestrogens in vitro on the growth of both normal and malignant breast cells.', 'Normal breast MCF-10A cells and breast cancer MCF-7 cells were incubated with 10 (-10)-10(-4)\u200a mol/l genistein, resveratrol, and quercetin (plasma concentrations in human: 10 nmol/l-10 μmol/l) for 48 h and were then extracted for a cell proliferation assay (MTT), and for a cell death assay (TUNEL) assay. The proteins involved in the proliferative and apoptotic pathways were evaluated by Western blot analysis. Additionally, a comparison with 17β-estradiol as well as an evaluation of the differential effects on estrogen receptors (ER) α and β were performed.', 'MCF-7 cell proliferation was significantly inhibited at the concentrations greater than 10(-4) mol/l for all three phytoestrogens and from 10 (-5) mol/l for resveratrol and quercetin. MCF-10A cell proliferation was significantly increased at the concentrations from 10 (-8) to 10 (-5) mol/l for genistein and resveratrol and only at 10 (-5) mol/l for quercetin. Apoptotic cells were significantly increased by these phytoestrogens in the MCF-7 cells. At a concentration of 10 (-7)\u200a mol/l of these phytoestrogens, a significant reduction of PI3K and Akt and an increase of Fas ligand, Fas-associated protein with death domain, cytochrome C, truncated Bid, caspase-9, and caspase-3 were noted in the MCF-7; PI3K and Akt were significantly increased in the MCF-10A. ERβ expression was significantly elevated in MCF-10A and MCF-7 with these phytoestrogens. The effects of estradiol on normal and malignant breast cells were completely opposite to those of phytoestrogens.'] Do phytoestrogens induce differential effects on both normal and malignant human breast cells in vitro?