Transforming growth factors (TGFs) were initially isolated from the conditioned medium of transformed cell lines through their ability to stimulate anchorage-dependent cells to form colonies in soft agar (1 ,2 ). The ability to proliferate in an anchorage-independent manner is still one of the best in vitro correlates with tumorigenicity. Subsequent studies demonstrated that the growth-promoting activity in the conditioned medium consisted of two unique peptides, TGF-α and TGF-β (3 –5 ). Depending on the indicator cell line used, soft-agar colony growth could occur when TGF-α and TGF-β (i.e., NRK cells) or TGF-β alone (i.e., AKR-2B cells) were added to the serum-containing medium (6 ,7 ). This review will focus on TGF-β and cellular systems capable of responding in vitro to its growth modulatory activity independent of additional factors.