The low density lipoproteins of cholesterol-fed African green monkeys were isolated by either preparative ultracentrifugation or agarose column chromatography, then radioiodinated and fractionated by agarose column chromatography. A marked variation was noted in the specific radioactivity of the column fractions regardless of the method used to obtain low density lipoproteins. The radioactivity in each fraction migrated with the beta-globluins, but when the fractions were mixed with carrier plasma, alpha-migrating radioactivity was also detected which was associated with the apo-A-I and apo-C of the high density lipoproteins. The uptake of these apoproteins by the high density lipoproteins reduced the variation in specific radioactivity among the low density lipoprotein fractions. By successive injections of the various 125I-labeled low density lipoprotein fractions into the same recipient, it was also possible to compare the kinetic behavior of the fractions. Large and small low density lipoprotein particles were metabolized differently, indicating that the low density lipoproteins from cholesterol-fed African green monkeys are heterogeneous.