Loss of sensitivity to growth inhibition by transforming growth factor (TGF)-beta is a phenomenon often observed in human epithelial tumor cells and is linked to malignant progression. We tested a panel of estrogen receptor (ER)-positive and -negative breast cell lines for their sensitivity to TGF-beta and a related member of the TGF-beta superfamily, activin. Both TGF-beta-sensitive (MCF7, Hs578T, and BT20) and -resistant (two T47D variants, ZR75-1, MDA-MB231, and MDA-MB468) cell lines were found, with no strict correlation between ER content and sensitivity to TGF-beta. In contrast, all four ER-positive cell lines were inhibited by activin A, whereas the ER-negative lines were not. To examine whether resistance to TGF-beta and activin resulted from the absence of the corresponding receptors, mRNA expression of the types I and II receptors was studied. TGF-beta receptor II was not expressed in the two T47D variants and was low in ZR75-1 cells. Upon stable transfection of the TGF-beta receptor II in one of the T47D variants, sensitivity to TGF-beta 1 and TGF-beta 2 was restored with respect to inhibition of anchorage-dependent and -independent proliferation, indicating that other signal transduction components are functionally intact. Sensitivity to TGF-beta in the transfectants was dependent on the expression level of the newly introduced receptor. Resistance to activin in the ER-negative cell lines could be explained in BT20 and Hs578T cells, but not in MDA-MB231 and MDA-MB468, by low activin receptor expression. These results show that resistance to TGF-beta and activin is often, but not always, due to reduced expression of the signaling receptor in breast cancer cells. The activin resistance of ER-negative breast tumor cells may be involved in their increased malignancy compared with ER-positive cells.