THE LIPOPROTEINS of human serum are conveniently defined and quantitated in the ultracentrifuge according to their flotation rates under carefully defined conditions.' Inasmuch as the concentration of certain serum lipoprotein molecules so defined (Sf 1220; Sf 20-100*) show a positive correlation with the incidence of human atherosclerosis,2 those factors which determine the lipoprotein spectrum of the serum in any individual become of potential importance in understanding the pathologic physiology of atherosclerosis. A search for determining factors important in lipoprotein metabolism logically includes several varied aspects of cholesterol metabolism. The interrelationships between serum lipoprotein spectra and oral cholesterol metabolism are under investigation in this study. Labeled cholesterol has been administrated to a series of 10 patients, and it has been possible to demonstrate a quantitative metabolic defect in lipid metabolism in patients who have abnormal ultracentrifugal lipoprotein spectra.