Insulin stimulates the growth and proliferation of a variety of somatic cells in culture, and evidence suggests that insulin is also an important regulator of growth in vivo. In cell culture, insulin interacts synergistically with other hormones and growth factors such as platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), epidermal growth factor (EGF), tumor-promoting phorbol esters, and thrombin, to stimulate progression through the cell cycle of cells that have been arrested in G1 by deprivation for serum. In addition, insulin is required by most cells for optimal long term growth in hormone-supplemented serum-free media. In some cells, such as human skin fibroblasts, the growth-promoting effects of insulin appear to be mediated primarily by its low affinity interaction with receptors for insulin-like growth factor I (IGF-I). In other cells, such as hepatocytes, hepatoma cells, adrenocortical tumor cells, mammary carcinoma cells, and F9 embryonal carcinoma cells, insulin appears to stimulate growth by binding to high affinity insulin receptors. The insulin and IGF-I receptor proteins, like the receptor proteins for other growth-promoting hormones such as EGF and PDGF, are closely associated with tyrosine-specific protein kinase activities. The mechanism by which the binding of insulin to its receptor and activation of the receptor-associated tyrosine protein kinase activity control intracellular protein phosphorylation and dephosphorylation reactions, such as the phosphorylation of ribosomal protein S6, is a subject of considerable current interest. The phosphorylation of ribosomal protein S6 may be related mechanistically to the activation by insulin of protein synthesis, and hence the passage of cells through the G1 phase of the cell cycle. Malignant transformation does not generally result in a total loss of the growth requirement of cells for insulin or insulin-like growth factors, although transformation is accompanied in some cases by a qualitative reduction in the insulin/IGF requirement. Abnormalities in insulin production or sensitivity in vivo are accompanied by abnormalities in growth; thus, insulin appears to be an important regulator of growth in vivo. Some of the growth-promoting effects of insulin in vivo may be attributable to direct action of insulin, while other effects may be caused by the regulatory effect of insulin on somatomedin production, and possibly on somatomedin action.