Conditioned medium (CM) from a human mammary carcinoma cell line, MCF-7, and ten individual clones derived from these cells was examined for the presence of transforming growth factors (TGFs). Concentrated CM from all of the MCF-7 cell lines was found to stimulate the anchorage-independent growth of normal rat kidney cells in soft agar and to inhibit the binding of epidermal growth factor (EGF) to mouse NIH/3T3 fibroblasts and to A431 human epidermoid carcinoma cell membranes. The soft agar stimulating activity was heat stable but sensitive to treatment with dithiothreitol. EGF receptors were measured on the MCF-7 cell lines to determine whether the amount of TGFs associated with the CM from the various cell lines was correlated with the level of EGF receptors being expressed on these cells. Moreover, the intrinsic cloning efficiency of these lines in soft agar was measured to ascertain if any correlation might exist between the level of TGFs associated with these cells and the ability of these cell lines to form colonies in soft agar. Although all the MCF-7 cell lines had approximately the same number of EGF receptors per cell, ranging from 3 to 6 X 10(3) sites/cell, CM from these lines varied in potency with respect to inducing the growth of normal rat kidney cells as colonies in soft agar and in inhibiting the binding of EGF to NIH/3T3 cells. Likewise, the level of TGFs associated with the CM from the various clones showed no correlation with the ability of these individual lines to grow as colonies in soft agar. TGF activity was also detected in acid-ethanol extracts prepared from MCF-7 cells propagated in nude mice as tumors and in the extracts from two transplantable human mammary adenocarcinomas, Clouser I and II. In addition, approximately 50% of the normal rat kidney colonies formed in response to the Clouser II tumor extracts exhibited a branching morphology in contrast to spherical colonies produced by Clouser I or MCF-7 extracts. These results demonstrate that human mammary carcinoma cells from both established cell lines and cells maintained in nude mice as tumors contain TGF-like activities. Furthermore, the variation in TGFs associated with the CM from the MCF-7 clones suggests that the parent MCF-7 cell line contains a heterogeneous population of cells.