Cholesterol is a membrane constituent widely found in biological systems which serves a unique purpose of modulating membrane fluidity, elasticity, and permeability. It literally fills in the gaps created by imperfect packing of other lipid species when proteins are embedded in the membrane. Cholesterol serves much the same purpose in model membranes. Unfortunately, cholesterol presents certain problems when used in human pharmaceuticals. High purity sources suitable for clinical applications are not widely available. Most cholesterol commercially available is derived from egg or wool grease (sheep derived). These animal sources are potentially not suitable for human pharmaceuticals due to the potential viral contamination. Also, cholesterol is readily oxidized creating a stability problem for lipid based drug products. Some of these oxidation by-products tend to be rather toxic in biological systems. The oxidation products 25-hydroxy cholesterol, 7-keto-cholesterol, 7α- and 7ß-hydroxycholesterol, cholestane-3ß,5α,6ß-triol and the 5- and 7-hydroperoxides, were found in a concentrate which had activity causing aortic smooth muscle cells to die. This suggests that results from studies on atherosclerosis involving feeding experimental animals a diet containing cholesterol stored under adverse conditions (room temperature, open to air) could be ambiguous due to the potential presence of significant quantities of oxidized sterols.