Transforming growth factors (TGFs) isolated from murine sarcoma virus-transformed 3T3 cells have been separated by high-pressure liquid chromatography into two subsets. One subset, called TGF alpha, competes with epidermal growth factor (EGF) for receptor sites, whereas the other, called TGF beta, does not. TGB beta, purified by high-pressure liquid chromatography, will not induce formation of large colonies of cells in soft agar in the absence of TGF alpha or EGF. However, the combined action of either TGF alpha or EGF (which by themselves are relatively ineffective in promoting growth of cells in soft agar) together with TGF beta results in a potent synergistic effect, with formation of large colonies. Chemically modified analogs of EGF also potentiate TGF beta activity to the extent that they bind to the EGF receptor. It is suggested that TGF beta may be an important mediator of the known effects of both TGF alpha and EGF on neoplastic transformation.