The oxidation of LDL by a number of different cell types has been studied and described. Cells may be able to initiate and seed peroxides in LDL but, in the absence of metals or peroxidase activity, may not be able to oxidize the lipoprotein. Monocyte-derived macrophages and neutrophils are able to initiate and propagate lipid peroxidation. The oxidative role of smooth muscle cells and endothelial cells may be restricted solely to the generation of hydrogen peroxide or other oxidants. The above rationale presents a problem in using LDL preparations that already contain peroxides, and in using media that generate peroxides independently of cells.