Myeloperoxidase is a lysosomal enzyme found in cells of the myeloid series which metabolises most of the hydrogen peroxide generated by activated phagocytes. It is a major constituent of azurophilic cytoplasmic granules that uses hydrogen peroxide to oxidise a variety of aromatic compounds and chloride ions to hypochlorous acid (HOCI), a strong oxidant. HOCI is the most bacteriocidal oxidant known to be produced by neutrophils. HOCI reacts with proteins to form cytotoxic chloramines. Myeloperoxidase is a major component in all myeloid cells, including mature granulocytes and is a superior marker to myeloperoxidase mRNA, whose level decreases with the maturation of the cell and is not detectable from the myelocyte stage onwards. Myeloperoxidase is expressed in neutrophil granulocytes and monocytes in blood, in precursors of granulocytes in the bone marrow and in Kupffer cells of the liver.