Background/Aim:Genistein, a soy component, has been shown to have a biphasic proliferative effect in breast cancer cells, inhibiting in vitro cell proliferation at high concentrations (>10 μmol/l), while stimulating cell proliferation at lower concentrations (<10 μmol/l). However, epidemiological studies have shown an inverse correlation between the intake of genistein and the incidence of breast cancer. One of the possible reasons for this discrepancy could be the differing status of the estrogen receptor (ERα and/or ERβ). Genistein selectively binds to ERβ with strong affinity and thereby could be a potential chemotherapeutic agent against breast cancer of the ERα-negative and ERβ-positive type. Therefore, the objective of the present study was to determine whether the proliferative effects of genistein were caused by its activity as a selective ERβ agonist or merely as an antiestrogen. Method: This study was carried out in MDA-MB-231 (ERβ) and T47D (ERα and ERβ) human breast cancer cells. Cell proliferation was determined by the MTT (3-[4,5-dimethyl-2-thiazolyl]-2,5-diphenyl-2H-tetrazolium bromide) assay. The cells were grown in estrogen-starved media and exposed to genistein at different concentrations for 72 h, either in the presence or absence of 17β-estradiol. Results: A significant decrease in cell proliferation was seen in MDA-MB-231 cells at low concentrations of genistein in the presence of 17β-estradiol, as compared to genistein alone. In T47D cells, which are known to have a predominance of ERα over ERβ, genistein showed a biphasic cell proliferative response both in the presence and absence of 17β-estradiol. Conclusions:Our results suggest that in cells with a predominance of ERα, genistein acts as an agonist to ERα, and in cells with ERβ alone, genistein most likely acts as an antiestrogen. Our results also suggest that genistein could be useful as a chemotherapeutic agent in premenopausal women with breast cancer of the ERα-negative and ERβ-positive type.