Binding of human low density lipoproteins (LDL) to their specific receptor on cultured cells can be inhibited by treatment with 1,2-cyclohexanedione which blocks a number of functionally significant arginyl residues on the apolipoprotein. We have used this observation to examine the role of the receptor pathway in LDL catabolism in man. The plasma clearance rates of 125I-LDL and 131I-cyclohexanedione-treated LDL were measured in four normal and four heterozygous familial hypercholesterolemic subjects. Chemical modification of the lipoprotein significantly reduced its fractional clearance rate and permitted calculation of receptor-mediated and receptor independent catabolism in both groups. The normal subjects cleared 11% of their plasma LDL pool (corresponding to 3.0 mg/kg per day) by a receptor-independent path. In tared daily by these pathways, respectively. Because the mean apoLDL pool size in the group was increased 3-fold over normal, this gave absolute clearance rates for the apoprotein of 2.5 mg/kg per day via the receptors and 12.8 mg/kg per day by the nonreceptor pathway. We conclude that the specific LDL receptor mechanism operates in vivo and probably accounts for 33% and 16% of overall LDL catabolism in normal and heterozygous familial hypercholesterolemic subjects, respectively.