The plasma of older human patients who have recovered from a myocardial infarction is shown to induce a significant increase above normal in the synthesis of fibrinogen in an in vitro system based upon human liver biopsy samples. The probable identity of the plasma factor is free fatty acid (FFA). Palmitate will induce up to a 9.45-fold increase in the uptake of [1-14C]glycine into the protein molecule. Short chain and unsaturated long chain fatty acids induce a lesser, but significant increase in the synthesis of fibrinogen. Introduction into the plasma of a young normal subject of an amount of stearate or palmitate equivalent to the difference in the plasma level of FFA between the normal and arteriosclerotic subjects raised the rate of in vitro synthesis of fibrinogen to the level noted in a mouse liver system bathed with the plasma of the arteriosclerotic patient. This biosynthetic control manifested by FFA appears to provide a connecting metabolic link between fat metabolism and blood clotting in the genesis of arteriosclerosis.