The ability of cells to proliferate independently of a surface substratum is a property that distinguishes transformed cells from normal cells. Current thinking is that this ability of tumor cells is brought about by the production in tumor cells of anchorage-independent growth conferring factors, or transforming growth factors (TGF). New observations that we and others have made indicate that normal tissues in addition to tumor tissues can make TGF. For example, we have found that TGF activities are made by or accumulate in proliferating bovine mammary gland, in rat, mouse and human adenocarcinomas and in fact are present in large amounts in human milk. The TGF activities in human milk and human mammary tumors have been partially purified. The major species from the two sources have identical PI's and are probably the same protein. Milk TGF has been purified about 2,000 fold using isoelectric focusing, HPLC gel permeation and reverse phase chromatography. The activity is similar to human EGF in its size, its insensitivity to heat and in its inactiviation by proteases and disulfide reducing agent. It differs from human EGF in its pI and in its potency in promoting anchorage independent growth of NRK cells