Patas monkeys were studied for 2 years on three dietary regimes: (1) commercial chow (control diet); (2) semipurified diet plus lard (fat-fed); and (3) semipurified diet plus lard and cholesterol (cholesterol-fed). The control and fat-fed animals had similar lipoproteins which were equivalent to the human very low density, low density (LDL), and high density lipoproteins. An additional lipoprotein referred to as LDL-II appeared to be equivalent to the human Lp(a). The cholesterol-fed animals developed accelerated atherosclerosis associated with a hypercholesterolemia which was characterized by (1) the appearance of beta-migrating lipoproteins (B=VLDL) in the d less than 1.006, (2) an increase in the intermediate lipoproteins and LDL, and (3) the appearance of LDL-II which contained a prominence of the arginine-rich apoprotein. The arginine-rich apoprotein was also a prominent component of the B-VLDL and intermediate lipoproteins. Characterization of this apoprotein revealed that it contained 11.5 mol % arginine, had a molecular weight of approximately 34 000, and coelectrophoresed with the arginine-rich apoprotein of man, dog, swine, rat, and rabbit.