The myeloperoxidase (MPO) system of activated phagocytes is central to normal host defense mechanisms, and dysregulated MPO contributes to the pathogenesis of inflammatory disease states ranging from atherosclerosis to cancer. MPO is by far the most abundant protein product in azurophilic granules of neutrophils (5%), constitutes approximately 1% of monocyte protein, and is found in the lysosomes of other polymorphonuclear leukocytes and macrophages. The phagosomal oxidative burst is initiated by a stimulus-dependent assembly of the phagocytic NADPH oxidase (Phox), a multimeric protein complex located on the phagosomal membrane. Phox then reduces molecular oxygen to produce superoxide anion (O2•-) which further dismutates to yield hydrogen peroxide (H2O2) (1). Upon phagocytic activation, large quantities of active MPO are secreted into phagosomes, catalyzing the production of highly bactericidal hypochlorous acid (HOCl) using H2O2 and chloride ions (Cl-) as substrates.