The sulfated glycosaminoglycan, heparin, was found to release 1* 5l-labeled low denslty lipoprotein (‘* 51-LDL) from its receptor site on the surface of normal human flbroblasts. Measurement of the amount of 1251, LDL released by heparln permltted the resolution of the total cellular uptake of 1* 5l-LDL at 37% Into two components: flrst, an inltlal rapid, high affinity blndlng of the llpoproteln to the surface receptor, from which the ‘* 5l-LDL could be released by heparln, and second, a slower process attrlbutable to an endocytosis of the receptorbound lipoprotein, which rendered It resistant to heparin release. At 4% the amount of heparlnreleasable ‘* 5l-LDL was similar to that at 37%, but interiorization of the llpoproteln did not occur at the lower temperature. The physiologic Importance of the cell surface LDL receptor was emphaslzed by the flndlng that mutant flbroblasts from a subject wlth homozygous Familial Hypercholesterolemla, which lack the abllity to take up ‘* 5l-LDL at 37%, did not show cell surface blndlng of ‘* 5l-LDL, as measured by heparln release, at either 4% or 37%. Although heparin released ‘* 5l-LDL from its bindlng she, it did not release 5H-concanavalin A from Its surface receptor, and conversely, IX-methyl-D-mannopyranoslde, which released 5H-concanavalin A, did not release surface-bound ‘251-LDL. When added to the culture medium slmultaneously with LDL, heparln prevented the binding of LDL to Its receptor and hence prevented the LDL-mediated suppresslon of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity. The uptake of LDL by fibroblasts is proposed as a model of receptor-mediated adsorptive endocytosls of macromolecules In human cells.