Human myeloperoxidase (MPO; EC 1.11.1.7) is a specific heme (Fe³⁺) peroxidase, present in high concentrations in the azurophilic granules of neutrophils. Its amino acid and genomic sequences have been elucidated, and recombinant MPO is produced from genetically engineered mammalian cells. This peroxidase has the unique activity of chlorination, generating hypochlorous acid (HOCl) from hydrogen peroxide and chloride anion, but also chlorine and monoatomic chlorine. By interacting with other enzymes of neutrophils and reacting with the products of neutrophil activation, MPO also produces other reactive oxygen species (singlet oxygen, hydroxyl radical, nitrosyl and nitryl chloride, etc.). In phagolysosomes, MPO acts together with NADPH oxidase and proteases for the destruction of the ingested organisms, by binding to the microorganism walls and producing locally HOCl, which is particularly active against the polysaccharidic capsules. MPO activity influences the transduction of the cellular signal (activation of NF-κB, chlorination of tyrosyl residues on essential enzymes, etc.) and modulates the functions of cells: it decreases the killer activity of NT lymphocytes and, after internalization, it enhances the microbial activity of macrophages. MPO is taken up by endothelial cells. MPO deficiency is the most common neutrophilic lysosomal enzyme deficiency, but usually without apparent increased susceptibility to infection or altered inflammatory response. MPO has been recognized to be responsible for the oxidation and chlorination of low density lipoproteins, contributing to the early stage of atherosclerosis. In disease with excessive and uncontrolled inflammatory reaction, MPO can be released in the extracellular milieu where it becomes cytotoxic for neighboring cells (oxidant stress) and oxidizes tissues and proteins (thiol oxidation, oxidation and chlorination of lipids and amino acids, etc.). Out of the neutrophil, the activity of MPO would be quickly inhibited by proteins; however, active MPO has been measured in broncho-alveolar lavage fluids from patients with acute lung injury. This specific enzyme, thus, presents a double role of essential host protection when acting into the phagocytes and of host damage when released in the extracellular milieu.