Over the last several years evidence has accumulated to support the model that cell growth is regulated by hormones and growth factors. A number of these growth factors have been characterized, including epidermal growth factor (EGF), fibroblast growth factor, platelet-derived growth factor, and transforming growth factors. 1-11 The growth regulatory properties of steroid hormones, prostaglandins, and their metabolites, as well as the retinoids, have also been the subject of intensive investigation. 12-24 A common property of these compounds is that they mediate their biological actions by binding to or modulating specific high-affinity receptor sites located on target cells. The precise biochemical events leading to alterations in cell growth following activation of these receptors is varied and depends on the nature of the growth regulatory substance and on the target cell.