The study investigates the dual control of cell growth in BALB/c 3T3 cells by somatomedins and platelet-derived growth factor (PDGF). Quiescent 3T3 cells exposed to PDGF become "competent" to replicate their DNA but do not "progress" into the S phase without growth factors in platelet-poor plasma. Hypophysectomized rat plasma lacks progression activity, as it does not stimulate DNA synthesis in PDGF-treated cells. Adding somatomedin C to hypophysectomized rat plasma restores progression activity, indicating that somatomedins are required for progression. Various growth factors were tested for their ability to induce competence or progression, with somatomedins and related polypeptides showing potent progression activity. EGF and Ca₃(PO₄)₂, which lack progression activity, function to induce competence. Simian virus 40 (SV40) provides both competence and progression activity, suggesting that SV40 disrupts both pathways. The study concludes that the control of 3T3 cell growth by competence factors (PDGF) and somatomedins may represent a common pattern for growth regulation in animal tissues.The study investigates the dual control of cell growth in BALB/c 3T3 cells by somatomedins and platelet-derived growth factor (PDGF). Quiescent 3T3 cells exposed to PDGF become "competent" to replicate their DNA but do not "progress" into the S phase without growth factors in platelet-poor plasma. Hypophysectomized rat plasma lacks progression activity, as it does not stimulate DNA synthesis in PDGF-treated cells. Adding somatomedin C to hypophysectomized rat plasma restores progression activity, indicating that somatomedins are required for progression. Various growth factors were tested for their ability to induce competence or progression, with somatomedins and related polypeptides showing potent progression activity. EGF and Ca₃(PO₄)₂, which lack progression activity, function to induce competence. Simian virus 40 (SV40) provides both competence and progression activity, suggesting that SV40 disrupts both pathways. The study concludes that the control of 3T3 cell growth by competence factors (PDGF) and somatomedins may represent a common pattern for growth regulation in animal tissues.