Lipoprotein oxidation is a key early stage in the development of atherosclerosis. Oxidation of low-density lipoprotein (LDL) is initiated by both enzyme-mediated and non-enzymic mechanisms in vivo, and oxidized LDL has many atherogenic properties. Oxidation of LDL in vivo is likely to be influenced by local environmental factors, such as pH. The composition of LDL is also important, including such factors as antioxidant content, fatty acid composition and particle size.