Significant differences among individuals occur in the lipoprotein response to atherogenic diets in cynomolgus and African green monkeys. The range of concentrations of total plasma cholesterol (TPC) was 100-600 mg/dl and of apolipoprotein (apo) E (quantified by enzyme-linked immunosorbent assay) was 3-20 mg/dl in the animal groups of this study. The correlation between the concentrations of TPC and of apo E was r = 0.89 in these animals. To determine which lipoprotein classes contained the majority of apo E, agarose gel-filtration chromatography was used to subfractionate whole plasma. In hypercholesterolemic monkeys, the majority of the apo E and apo B-100 coeluted within the region of low density lipoprotein (LDL). In normocholesterolemic monkeys, the majority of apo E coeluted with apo A-I and high density lipoproteins. A strong positive correlation was seen between the concentrations of plasma apo E and LDL cholesterol (r = 0.9), but there was no significant correlation between high density lipoprotein apo E and either TPC or plasma apo E concentrations. A positive correlation of r = 0.8 was found between the LDL apo E to apo B-100 molar ratio and the average LDL particle size, suggesting an increase in the number of apo E molecules on the larger LDL particles. Within individual animals, LDL were heterogeneous and the LDL subfractions of lower density (1.02 less than d less than 1.03 g/ml) had the highest proportion of apo E, although apo E was present on LDL of all densities. A strong positive correlation between plasma apo E concentration and coronary artery atherosclerosis was identified, and in stepwise regression analysis, plasma apo B concentration and the apo E to apo B molar ratio of LDL together accounted for more than 90% of the variation in the atherosclerosis end point of coronary artery intimal area. These data strongly suggest that the enrichment of LDL with cholesteryl esters and apo E, which occurs in hypercholesterolemic primates, is an atherogenic feature of the plasma lipoproteins.