The extent of triglyceride lipolysis by postheparin lipemia clearing factor (lipoprotein lipase) was studied in vitro in 8 atherosclerotic individuals. Varying doses of heparin were injected intravenously and subcutaneously and blood was drawn at stated time intervals for analysis. The rate of splitting of human low density lipoproteins was then determined by measurement of the rate of release of unesterified fatty acids upon incubation of postheparin plasma plus lipoproteins in vitro at 37 C. The data obtained gives a range of the minimum in vivo removal of alimentary neutral fat following an injection of heparin.