The myeloperoxidases possessing the specific activity of 500 000-700 000 of o-dianisidine units per 1 mg of protein were isolated and purified from the lysosome-like granules of pig and cattle neutrophylic leukocytes. The absorbance spectra for the enzymes in the visible region are identical; their molecular weight is 140 000-160 000. The enzymes are made up of two subunits, each containing a heavy (m. w. 68 000) and a light (m. w. 10 000) polypeptide chains. The pH optimum for myeloperoxidase oxidation of o-dianisidine lies around 5.8-6.2 for both enzymes. The dependence of the reaction rate on H2O2 concentration does not obey the Michaelis--Menten kinetics. The optimal concentrations of the reaction substrates are 0.34 mM for o-dianisidine and 0.075 mM for H2O2. The amino acid composition and the peptide maps for pig and cattle myeloperoxidases are in many ways similar, showing no coincidence of enzymes, however, in terms of antigenic determinants. The similarities of some physico-chemical properties of pig and cattle enzymes and their immunological differences are discussed.