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Serum high- and low-density lipoproteins were spread between heptane and phosphate buffer. The cholesterol from both protein films became completely soluble in the heptane phase. Other neutral lipids were also found in the heptane, but phospholipids could not be detected. Shaking of whole serum or isolated lipoproteins with heptane resulted in only slight extraction of cholesterol by the fat solvent. The addition of albumin to the lipoproteins reduced the amount of cholesterol extractable by heptane. It is postulated that when aqueous solutions of lipoproteins are shaken with nonpolar fat solvents the degree of cholesterol extraction depends on the disruption of the lipoprotein structure through contact with various interfaces, and that other proteins may protect the lipoprotein by competing for position at the interface.