The study investigates the dual control of cell growth by somatomedins and platelet-derived growth factor (PDGF) in BALB/c 3T3 cells. Quiescent cells exposed to PDGF become "competent" to replicate DNA but require progression factors in plasma to enter the S phase. Hypophysectomized rat plasma lacks progression activity, but adding somatomedin C restores it, indicating somatomedin C is essential for progression. Growth factors like PDGF and fibroblast growth factor (FGF) induce competence, while somatomedins and other factors like MSA and insulin-like activity mediate progression. Growth factors with strong progression activity have little competence activity and vice versa. Simian virus 40 (SV40) induces both competence and progression. The study shows that somatomedins regulate progression through the G0/G1 phase of the cell cycle, while PDGF induces competence. The results suggest that the control of cell growth by competence factors and somatomedins is a common pattern in animal tissues. The study also highlights the role of somatomedins in growth regulation, as they are modulated by pituitary growth hormone and are required for progression. The findings indicate that somatomedins and PDGF play distinct roles in cell growth regulation, with somatomedins being essential for progression and PDGF for competence. The study provides a quantitative assay for progression activity and demonstrates that SV40 disrupts both competence and progression pathways.The study investigates the dual control of cell growth by somatomedins and platelet-derived growth factor (PDGF) in BALB/c 3T3 cells. Quiescent cells exposed to PDGF become "competent" to replicate DNA but require progression factors in plasma to enter the S phase. Hypophysectomized rat plasma lacks progression activity, but adding somatomedin C restores it, indicating somatomedin C is essential for progression. Growth factors like PDGF and fibroblast growth factor (FGF) induce competence, while somatomedins and other factors like MSA and insulin-like activity mediate progression. Growth factors with strong progression activity have little competence activity and vice versa. Simian virus 40 (SV40) induces both competence and progression. The study shows that somatomedins regulate progression through the G0/G1 phase of the cell cycle, while PDGF induces competence. The results suggest that the control of cell growth by competence factors and somatomedins is a common pattern in animal tissues. The study also highlights the role of somatomedins in growth regulation, as they are modulated by pituitary growth hormone and are required for progression. The findings indicate that somatomedins and PDGF play distinct roles in cell growth regulation, with somatomedins being essential for progression and PDGF for competence. The study provides a quantitative assay for progression activity and demonstrates that SV40 disrupts both competence and progression pathways.