Myeloperoxidase was found to promote peroxidation of phospholipids under acidic conditions in the presence of hydrogen peroxide and iodide ions. The peroxidation was markedly enhanced by pyrophosphate-chelated ferric iron and was inhibited by desferrioxamine and superoxide dismutase. This observation adds lipid peroxidation to the oxidative damage caused by myeloperoxidase, which is a phagocytic cell enzyme involved in phagocyte-mediated cell destruction.