Platelet-derived growth factor (PDGF) is a basic (pI congruent to 10) 30 000 molecular weight protein circulating in normal blood sequestered within the platelet alpha-granules. It binds with high affinity (Kd = 10(-11) M) to a specific cell-surface receptor found on many connective tissue cell types in culture. It is active in stimulating the metabolism and multiplication of connective tissue cells at very low concentrations (ED50 = 10(-11) M). It is likely that PDGF is released from platelets at sites of vascular damage and that it contributes toward the cell proliferation and connective tissue formation seen in healing wounds and in arteriosclerotic lesions. PDGF which does not bind to responsive cells at the wound site is largely inactivated by a plasma binding protein and is rapidly cleared from the circulation.