Fibroblasts cultured from the skin of subjects with homozygous familial hyperlipoproteinemia (HFH) internalize and degrade low density lipoproteins at a much lower rate than do fibroblasts from normal subjects. Evidence has been presented that this reflects the absence from such mutant cells of specialized binding sites with high affinity for low density lipoproteins. The specificity of this membrane defect in familial hypercholesterolemia is further supported by the present studies comparing the metabolism of low density lipoproteins (LDL) and high density lipoproteins (HDL) in normal fibroblasts and in fibroblasts from HFH patients. The surface binding (trypsin-releasable (125)I) of (125)I-labeled LDL by HFH cells was approximately 30% of that by normal cells at a concentration of 5 micro g LDL protein per ml. At the same concentration the internalization (cell-associated (125)I after trypsinization) and degradation (trichloroacetic acid-soluble non-iodide (125)I) of (125)I-labeled LDL were less than 10% of the values obtained with normal cells. In contrast, the binding of (125)I-labeled HDL to HFH cells was actually somewhat greater than that to normal cells. Despite this, the internalization and degradation of (125)I-labeled HDL by HFH cells averaged only 70% of that by normal cells. [(3)H]- or [(14)C]Sucrose uptake, a measure of fluid uptake by pinocytosis, was similar in normal and HFH fibroblasts. These findings are consistent with the proposal that fibroblasts from subjects with HFH lack high-affinity receptors for LDL. These receptors do not play a significant role in HDL binding and uptake. Instead, as previously proposed, HDL appears to bind randomly on the cell surface and its internalization is not facilitated by the specific mechanism that internalizes LDL. The small but significant abnormalities in HDL binding and internalization, however, suggest that there may be additional primary or secondary abnormalities of membrane structure and function in HFH cells. Finally, the observed overall rate of uptake of LDL (that internalized plus that degraded) by HFH fibroblasts was considerably greater than that expected from fluid endocytosis alone. This implies that adsorptive endocytosis, associated with binding to low-affinity sites on the cell surface, may play a significant role in LDL degradation by HFH cells, even though it does not regulate endogenous cholesterol synthesis in these cells.