The release of fatty acid from lipoproteins incubated with plasma obtained from a human after intravenous heparin administration is not significantly affected by sulfhydryl reagents, aerobiosis, anaerobiosis, alpha-naphthol, adenosine triphosphate, coenzyme A, but is inhibited by unesterified fatty acid and diisopropylfluorophosphate. The latter inhibition suggests that the active agent in plasma is an esterase. The Sf 6 lipoprotein is not broken down by the enzyme, nor is the serum tributyrinase activity enhanced significantly after heparin injection.