The larger lipoproteins of Sf greater than 10 also contain triglycerides. The action of lipases on these lipoproteins, either heparin-induced clearing factor, pancreatic lipase, or gastric lipase, yields fatty acids, glycerol, and smaller lipoproteins. Crude clearing factor attacks monoglycerides and triglycerides and is inactiveagainst several esters which plasma esterases attack. Albumin, plasma esterases, and probably other components are needed for optimal clearing factor activity. Low concentrations of diisopropyl fluorophosphate (DFP), a substance inhibiting most hydrolytic enzymes, inactivate crude clearing factor. Stoicheometric calculations made from this experiment indicate further the enzymatic nature of clearing factor. Evidence that clearing factor is different from other lipases and from esterases found in plasma is presented. Studies with purified lipoproteins are needed to determine what alterations, if any, occur in smaller lipoproteins. The separation and purification of lipoproteins by chemical and ultracentrifugal means are described. Whole human plasma separated by Cohn's methods 6 and 9 serves as starting material for the preparation of α1 α2 β and Sf 10 and greater lipoproteins. Studies of lipoproteins may give clues to the varied diseases associated with deranged lipid metabolism, such as xanthomatosis and atherosclerosis.