Transformed cells differ in many respects from their normal counterparts. Apart from their tumorigenic properties in vivo, transformed cells show an increased nutrient requirement and an enhanced rate of hexose uptake. In addition they have a reduced serum dependence, and can be cultured relatively simply in defined serum-free media. Normal rat kidney (NRK) fibroblasts, clone 49F, have been widely used as a nontransformed indicator cell line for studying the role of polypeptide growth factors in phenotypic transformation. Besides induction of anchorage-independent proliferation, phenotypic transformation of normal cells is also characterized by the loss of density-dependence of proliferation. This so-called contact inhibition process is one of the most obvious differences upon in vitro culturing of normal and transformed cells. Experimentally, studies on the effects of polypeptide growth factors on density-dependent proliferation of NRK cells are strongly facilitated by the observation that these experiments can readily be performed under serum-free conditions.