Myeloperoxidase (MPO) is a heme enzyme that is localized in azurophilic (primary) granules of myeloid cells and its synthesis occurs at an early stage of differentiation. The mature myeloperoxidase is a tetramer composed of two light (12 kDa) and two heavy (60 kDa) chains. This enzyme uses hydrogen peroxide to oxidize numerous substrates, including serotonin, melatonin or chloride, to produce reactive free radicals that contribute to immune reactions of myeloid cells against pathogens. Myeloperoxidase functions not only in host defense by mediating efficient microbial killing but also can contribute to progressive tissue damage in chronic inflammatory states such as atherosclerosis or acute pancreatitis.