The metabolism of apolipoproteins B-48 and B-100 (apo B-48 and B-100) in large triglyceride-rich lipoproteins was studied in three adults with familial dysbetalipoproteinemia (F. dys.) and compared to that of normolipidemic subjects. One Caucasian F. dys. subject was apparently homozygous for the common form of apo E-2, (Arg158----Cys), whereas the two Black subjects were homozygous for a different apo E-2 mutant (Arg145----Cys), which displays much less defective binding to cells than apo E-2 (Arg158----Cys). The lipoproteins were labeled with 125I and injected intravenously into fasted recipients. The results indicate that the terminal catabolism of triglyceride-rich lipoproteins of intestinal and hepatic origin is markedly impaired in apo E2/2 homozygotes with alleles Arg158----Cys and Arg145----Cys; despite long residence times, apo B-48 of chylomicrons and apo B-100 of large very low density lipoproteins are not converted appreciably to intermediate or low density lipoproteins in apo E2/2 homozygotes.