As a group of biologically occurring substances, lipids play a role ofmajor importance in the life process. Much attention in recentyears has been focused upon blood lipids, yet no satisfactory description or methods for study of these lipids in their native state have been given. Themajor serum lipids consisting of neutral fats, fatty acids, cholesterol and its esters, and phospholipids are present in total concentration of approximately 500 mg. per cent in the normal adult human. However, essentially none of these lipids circulate in the blood stream individually, but are present in theform of complex giant molecules ranging in molecular weight from approximately 200,000 to many million. In most chemical extraction techniques the identity ofeach of these molecular species is destroyed and the resulting analysis is a mere tabulation of the various chemically defined lipids that are present. For several years the existence in human sera of lipid-bearing macromolecules has been indicated by such workers as McFarlane (5) and Pedersen (7). In their ultracentrifugal studies of serum and serum fractions the presence in human sera of a low-density lipoprotein (the so-called X protein) was found to depend upon the concentration of both the serum proteins and the salt. These data led to a postulation of labile complexes of serum proteins with serum lipids. Additional information concerning serum lipoproteins was forthcoming during the war, when Cohn and coworkers (1), using low-salt, low-temperature, ethanol fractionation techniques were able to isolate two distinct lipoproteins from pooled human plasma. These were the ai-and ft-lipoproteins (6). The/31-lipoprotein weighed over one million molecular weight units and consisted of 75