Low density lipoprotein (LDL) and four subfractions, A, B, C, and D of very low density lipoprotein (VLDL), characterized by the following decreasing Sf values, greater than 400, 175-400, 100-175, and 20-100, respectively, were isolated by density gradient preparative ultracentrifugation from serum of normal men and women and analyzed for lipids, total protein, and apoB. The percentage distribution of the triglycerides of VLDL on fractions A to D were 1, 25, 30, and 45%, respectively, for both males and females. The numbers of VLDL particles for males in fractions B to D were 5, 10, and 40 X 10(12) per ml serum, respectively. Females, who had lower VLDL concentrations than males, had half as many particles per ml of serum. The number of LDL particles was similar in males and females and was 10 times that of VLDL. The relative composition of the lipoproteins changed progressively from fraction B to D and from D to LDL. The percentage of triglyceride and soluble proteins fell, due to loss of mass of these constituents from the particles. The percentage of apoB and cholesteryl esters rose; this was not due to an increase of mass but because the particles became smaller. There were no sex differences. The numbers of molecules per particle of the constituents showed a successive decrease from VLDL-B to LDL for triglycerides, free cholesterol, phospholipids, and soluble apolipoproteins, while it remained constant for apoB, as well as for cholesteryl esters from VLDL-C to LDL. These data fit the following hypothesis for the VLDL to LDL cascade. The large VLDL particle successively loses molecules of triglycerides (core) and of soluble protein, free cholesterol, and phospholipids (surface). In this process, VLDL retains its molecules of apoB, and cholesteryl esters are lost from large VLDL but not from medium or small-sized VLDL.