The effect of an atherogenic diet on serum apo E levels and distribution among the lipoproteins of rhesus monkeys was studied. Animals able to maintain their serum cholesterol levels below 250 mg/dl (hyporesponders) showed no significant change in their serum apo E levels; however, in monkeys whose serum cholesterol concentrations ranged from 250 to 850 mg/dl, serum apo E levels appeared to have increased in direct proportion to plasma cholesterol concentration (r2 = 0.92) such that in monkeys whose serum cholesterol concentration exceeded 650 mg/dl (hyperresponders), the apo E levels had increased 5-6-fold. The majority of the apo E (60%) in hyporesponders consuming the atherogenic diet was associated with HDL, whereas only 10% of the serum apo E was associated with HDL in hypercholesterolemic hyperresponders. Nonetheless, the absolute amount of HDL-associated apo E was the same in both phenotypes. Thus, essentially all of the increase in serum apo E levels in hyperresponders was due to an increase in non-HDL-associated apo E. The mean density of the fraction showing the greatest increase in apo E, and accounting for the majority of the d less than 1.063 g/ml apo E in hyperresponders, was 1.010 g/ml. That fraction was distinct from the lipoproteins principally responsible for the increase in apo B and cholesterol levels in those animals. The latter were smaller in size and higher in density than the major apo E-rich fraction. Nonetheless, the d less than 1.063 g/ml apo E apparently circulates on apo B-containing particles, since it was retained on an anti-apo B immunoaffinity column. These data show that a diet-induced hypercholesterolemia is accompanied by a marked increase in serum apo E levels in rhesus monkeys, but that the lipoproteins principally responsible for the increase in apo E levels are distinct from those mainly responsible for the hypercholesterolemia. They also suggest that the levels of apo E-containing HDL in hypercholesterolemic hyperresponders are not significantly lowered by the diet, even though those animals' apo A-I levels were severely reduced. Thus, both the LDL and the HDL of the hypercholesterolemic primate contain apo E-rich subfractions which are metabolically distinct from the principal lipoprotein family in each fraction.