Myeloperoxidase from leucocytes of peritoneal exudate was isolated by chromatography on DEAE-Sephadex, CM-cellulose and gel filtration on Sephadex G-75. The preparation obtained was homogenous as shown by repeated gel filtration and ultracentrifugation. The enzyme was purified 302-fold, with 16% yield. The sedimentation constant was equal to 4.05 S. Molecular weight of the enzyme, determined by gel filtration on Sephadex G-100, constituted 135,000 daltons. Myeloperoxidase had two pH optima of activity--at pH 4.6 and at pH 7.6. Km for o-dianizidine was equal to 4.0-10(-3)M and for hydrogen peroxide--to 1.65-10(-4)M. Myeloperoxidase of mice leucocytes differed from the enzymes of leucocytes from other animal species in its distinctly lower sedimentation constant and thermolability.