A new class of polypeptide hormones known collectively as growth factors has been identified. These polypeptides are able to stimulate DNA synthesis and mitosis of cells cultured in vitro. Growth factors have been isolated from several sources, including platelets, submaxillary glands, pituitary, brain, and medium conditioned by cells grown in vitro. Growth factors appear to behave like classic polypeptide hormones. In the cases of epidermal growth factor and nerve growth factor, specific cell membrane receptors have been studied in detail and partially purified. Platelet-derived growth factor and somatomedin-C interact synergistically and apparently, sequentially to promote cell proliferation in vitro. These studies suggest that cell proliferation is controlled by a synergistic interaction among several growth factors and, perhaps, other hormones. Several in vivo roles for growth factors and a role for tumor-cell-derived growth factors in neoplasia have been suggested.