Myeloperoxidase purified from leucocytes of purulent human sputum which is >98 percent pure. It is a lysosomal protein stored in azurophilic granules of the neutrophil which produces hypochlorous acid from hydrogen peroxide and chloride anion during the neutrophil respiratory burst. Supplied as a lyophilized, salt-free, green soluble powder which is stable for over a year when stored at 5 C. 150-160 units per mg of protein. 1 unit will decompose 1 umole of H2O2 per minute at pH 7.0 and 25 C using 4-amino-antipyrine as hydrogen donor. Recommended assay buffer: 0.1 M KPO4; pH 7.0.