A method for the analysis of serum lipoproteins is described. Lipoprotein fractions are isolated by repeated ultracentrifugations after progressively raising the solvent density. The separated fractions are characterized by lipid and protein analysis. It is possible to separate several lipoprotein fractions from the same serum sample. Routinely three fractions of densities <1.019, 1.019–1.063, and >1.063 have been analyzed. The accuracy of the procedure is limited by the accuracy of the chemical methods employed. Data obtained in young adults demonstrate that approximately two-thirds of the serum cholesterol and half the lipid phosphorus are contained in the density 1.019–1.063 fraction. The quantity of lipid is greater in the density <1.019 fraction and less in the density >1.063 fraction in young men as compared with young women. The ratio of total cholesterol to phospholipid is similar to that of serum in the density <1.019 fraction, considerably greater in the density 1.019–1.063 fraction, and considerably less in the density >1.063 fraction. In disease, hyperlipoproteinemia involves one or both of the two lower density fractions. The density >1.063 fraction is variably affected; in biliary obstruction it is markedly reduced. Alterations in composition of the lipoprotein fractions also occur; these are most striking in biliary obstruction. In animal sera wide variations in the distribution and composition of lipoproteins occur. A greater proportion of the lipid is associated with the density >1.063 fraction than in man. Subfractionation of the density >1.063 fraction demonstrates two fractions containing cholesterol and lipid phosphorus of densities 1.063–1.125 and 1.125–1.21. The fraction of density >1.21 contains 10 to 15 per cent of the serum lipid phosphorus but practically no cholesterol. This phosphorus is non-dialyzable and migrates with the α₁-albumin fraction in starch electrophoresis.A method for the analysis of serum lipoproteins is described. Lipoprotein fractions are isolated by repeated ultracentrifugations after progressively raising the solvent density. The separated fractions are characterized by lipid and protein analysis. It is possible to separate several lipoprotein fractions from the same serum sample. Routinely three fractions of densities <1.019, 1.019–1.063, and >1.063 have been analyzed. The accuracy of the procedure is limited by the accuracy of the chemical methods employed. Data obtained in young adults demonstrate that approximately two-thirds of the serum cholesterol and half the lipid phosphorus are contained in the density 1.019–1.063 fraction. The quantity of lipid is greater in the density <1.019 fraction and less in the density >1.063 fraction in young men as compared with young women. The ratio of total cholesterol to phospholipid is similar to that of serum in the density <1.019 fraction, considerably greater in the density 1.019–1.063 fraction, and considerably less in the density >1.063 fraction. In disease, hyperlipoproteinemia involves one or both of the two lower density fractions. The density >1.063 fraction is variably affected; in biliary obstruction it is markedly reduced. Alterations in composition of the lipoprotein fractions also occur; these are most striking in biliary obstruction. In animal sera wide variations in the distribution and composition of lipoproteins occur. A greater proportion of the lipid is associated with the density >1.063 fraction than in man. Subfractionation of the density >1.063 fraction demonstrates two fractions containing cholesterol and lipid phosphorus of densities 1.063–1.125 and 1.125–1.21. The fraction of density >1.21 contains 10 to 15 per cent of the serum lipid phosphorus but practically no cholesterol. This phosphorus is non-dialyzable and migrates with the α₁-albumin fraction in starch electrophoresis.