Low-density lipoprotein isolated from the yolk of hen's eggs contained 86% lipid. Approx. 19% of the total lipids were extracted by diethyl ether from a solution of native low-density lipoprotein in 1.0 M NaCl solution. Treatment with urea, guanidine hydrochloride, sodium thioglycolate, or non-ionic detergents did not increase the amount of lipid extracted, but treatment with sodium dodecylsulfate or sodium deoxycholate increased lipid extracted by ether to 50-64%. Digestion of low-density lipoprotein with phospholipase C increased lipid extracted to 74%, but phospholipase D digestion had no effect. Ether extracted 98% of the lipid from trypsin-digested low-density lipoprotein, and this lipid was composed of 73% glycerides, 7% sterols, and 21% phosphatides compared to 36% glycerides, 9% sterols, and 52% phosphatides in the residue lipid. Trypsin released 84% of the protein as soluble peptides. Further digestion of the residue with papain released lipids and peptides, and digestion of that residue with pronase also released lipids and peptides. The residue left after pronase digestion contained 27% protein, 39% lipid, 1% carbohydrate, and 33% of unknown material. The data obtained support the view that the low-density lipoprotein is a sphere of lipid surrounded by a layer of protein and phospholipid with hydrophobic groups on the inside and hydrophylic groups oriented to the outside making it very soluble in water and insoluble in organic solvents. © 1968.