The postprandial responses of ape B48, B100, E and lipids in triglyceride-rich lipoproteins (TRL) to a meal containing one-third of daily energy (39% fat calories) were compared in normolipidemic young men with ape E3/3 and apo E4/3 phenotypes, After the two groups consumed a diet rich in polyunsaturated fat for 15-29 d, their postabsorptive concentrations of TRL triglycerides, ape B48, and ape B100 were virtually identical, but their postprandial responses differed. In both groups, TRI, apo B48 increased at 3 h but returned to postabsorptive values at 6 h only in the ape E3/3 group; in the ape E4/3 group the concentration of ape B48 at 6 h was 80% higher than postabsorptive values. TRL apo B100 also increased at 3 h in the two groups and fell to postabsorptive values at 6 h in the ape E3/3 group but remained 51% higher than postabsorptive concentrations in the apo E4/3 group; this response was closely coupled to that of TRL cholesterol and ape E, These observations suggest that clearance of intestinal and hepatogenous TRL remnants is impaired in young men with an apo E4/3 phenotype.