Resveratrol (3,5,4'-trihydroxy-trans-stilbene), a phytoalexin, is a constituent of the human diet that has been shown to inhibit cellular processes associated with tumor initiation, promotion and progression. In this study, we examined the effect of synthetic resveratrol on the proliferative capacity of immortal and neoplastic human breast epithelial cells in culture. MCF-7, an estrogen receptor-positive breast cancer cell line, MCF-10F, an immortal estrogen receptor-negative breast epithelial cell line, and MDA-MB-231, a malignant estrogen receptor-negative breast epithelial cell line, were treated with 5, 10, 20 or 40 microg/ml resveratrol, and their proliferative activities were determined with the WST-1 colorimetric assay after periods of time ranging from 24 to 144 h of treatment. Our results showed that this phytoalexin inhibited the proliferation of human breast epithelial cells in a dose- and time-dependent manner. Treatment of cells with resveratrol reduced the number of viable cells and prevented the exponential growth of the three cell lines examined. These observations indicate that resveratrol has a direct antiproliferative effect on human breast epithelial cells that is independent of the estrogen receptor status of the cells. Thus, this dietary compound is a potential chemopreventive agent for both hormone responsive and non-responsive breast cancers.