The metabolism of low density lipoprotein (LDL, beta lipoprotein) was studied in 10 normal individuals and 10 patients with familial type II hyperlipoproteinemia using purified radioiodinated LDL. Over 97% of the label was bound to the protein moiety of LDL and therefore the turnover data reflect the fate and distribution of LDL-apoprotein. Comparison of the metabolic behavior of biologically screened and unscreened labeled LDL preparations in dogs as well as the analysis of the urinary excretion of radioiodide derived from labeled LDL degradation in humans indicated that no significant denaturation resulted from the isolation, purification, and labeling techniques. The plasma concentration of LDL-cholesterol in normals was 105+/-21 mg/100 ml (mean +/-1 SD) in contrast to 254+/-47 mg/100 mg in patients with type II hyperlipoproteinemia; these values corresponded to LDL-apoprotein concentrations of 63+/-13 mg/100 ml and 153+/-30 mg/100 ml, respectively. Despite these differences in concentration, the synthetic rate of LDL-apoprotein in both groups was not significantly different (14.43+/-1.75 mg/kg per day in normals vs. 15.01+/-1.71 mg/kg per day in type II) nor was there any difference in the fraction of the total exchangeable LDL which was in the intravascular space (68.4+/-4.3% vs. 73.3+/-5.2%). However, the fractional catabolic rate of LDL in normal individuals differed significantly from that of patients with type II hyperlipoproteinemia (0.462+/-0.077/day in normals vs. 0.237+/-0.044/day in type II) and correspondingly the biological half-life of LDL was significantly prolonged (3.08+/-0.35 days normals vs. 4.68+/-0.44 days in type II). These data indicate that the pathologic elevation of plasma LDL concentration in the individuals with type II hyperlipoproteinemia studied here is due to a decreased fractional rate of LDL degradation rather than to an abnormality of LDL synthesis. This defect of catabolism may be the primary defect in type II hyperlipoproteinemia or, alternatively, may be secondary to an underlying abnormality in lipid metabolism.