The two major isoflavones in soy, genistein and daidzein, are well known to prevent hormone-dependent cancers by their anti estrogenic activity. The exact molecular mechanisms for the protective action are, however, not provided yet. It has been reported that genistein and daidzein have a potential anticancer activity through their antiproliferative effect in many hormone-dependent cancer cell lines. Transforming growth $factor-eta1(TGF-eta1)$ has also been found to have cell growth inhibitory effect, especially in mammary epithelial cells. This knowledge led to a hypothetical mechanism that the soy isoflavones-induced growth inhibitory effect can be derived from the regulation of $TGF-eta1$ and $TGF-eta$ receptors. In order to test this hypothesis, the effects of the soy isoflavones at various concentrations and periods on the expression of $TGF-eta1$and $TGF-eta$ receptors were investigated by using Northern blot analysis in human breast carcinoma epithelial cell lines, an estrogen receptor positive cell line (MCF-7) and an estrogen receptor negative cell line (MDA-MB-231). As a result, only genistein has shown a profound dose-dependent effect on $TGF-eta1$ expression in the $ER^+$ cell line within the range of doses tested, and the expression levels are correspondent to their inhibitory activities of cell growth. Moreover, daidzein showed down-regulated $TGF-eta1$ expression at a low dose, the cell growth proliferation was promoted at the same condition. Therefore, antiproliferative activity of the soy isoflavones can be mediated by $TGF-eta1$ expression, and the effects are mainly, if not all, occurred by ER dependent pathway. The expression of $TGF-eta$ receptors was induced at a lower dose than the one for $TGF-{eta}1$ induction regardless of the presence of ER, and the expression patterns are similar to those of the cell growth inhibition. These results indicated that the regulation of $TGF-eta$ receptor expression as well, prior to $TGF-eta1$ expression, may be involved in the antiproliferative activity of soy isoflavones. Little or no expression of $TGF-eta$ receptors was found in the MCF-7 and MDA-MB-231 cells, suggesting refractory properties of the cells to growth inhibitory effect of the $TGF-eta$. The soy isoflavones can seemingly restore the sensitivity of growth inhibitory responses to $TGF-eta1$ by re-inducing $TGF-eta$ receptors expression. In conclusions, our findings presented in this study show that the antitumorigenic activity of the soy isoflavones could be mediated by not only $TGF-eta1$induction but $TGF-eta$ receptor restoration. Thus, soy isoflavones could be good model molecules to develop new nonsteroidal antiestrogenic chemopreventive agents, associated with, regulation of $TGF-eta$ and its receptors.