Myeloperoxidase (MPO) is a major neutrophil protein and is also present in some monocyte populations. Neuutrophil MPO, is stored in azurophilic granules and released during phagocytosis. It is a heme enzyme utilizing superoxide and hydrogen peroxide to produce hypochlorous acid (HOCL) and other reactive oxidants. HOCL thusly produced is capable of destroying bacteria however it also plays a key role in various inflammatory pathologies, such as cystic fibrosis rheumatoid arthritis and atherosclerosis where the net result is additional tissue damage. For its role in atherosclerosis, MPO is being heavily investigated a promising new cardiovascular biomarker.