The concentration of individual plasma apoproteins together with the in vivo metabolism of 125I-labelled high density lipoprotein was studied in an adult male patient with abetalipoproteinemia. Plasma concentrations of cholesterol (26 mg/dl) and triglyceride (5 mg/dl) were markedly reduced and apoprotein B was absent. The concentrations of apo AI, apo All, CI, CII, Oiland E were also lower than in normal subjects. To further evaluate these changes we studied the in vivo metabolism of autologous 125I-labelled HDL over a period of 13 days. The plasma half-life (4.1 days) and fractional catabolic rate (0.23) were similar to previously reported values for normal subjects, but overall synthesis of apoproteins AI (5.7 mg/kg/day) and All (3.6 mg/kg/day) were significantly reduced. After separation of delipidated HDL apoproteins by polyacrylamide-gel electrophoresis we were unable to demonstrate any differences in the half-life or fractional catabolic rates of apoproteins AI and All and CII plus OIL We conclude that the low concentrations of apoproteins AI and All in the plasma of our patient with abetalipoproteinemia are a consequence of reduced synthesis and do not reflect enhanced catabolism.