The kinetic properties and thermal stability of myeloperoxidase from mouse peritoneal leukocytes were investigated. The linear nature of the dependence of the rate of the myeloperoxidase reaction on the concentration of the enzyme was demonstrated. Graphs of the dependence of the reaction rate on the concentration of substrates (hydrogen peroxide and o dianisidine) differed from hyperbolas; moreover, for each of the substrates an effect of substrate inhibition was observed. The deviation of the curves from the Michaelis Menten hyperbolas, possibly due to nonequivalence and interaction of the subunits comprising the enzyme molecule, was characterized by a variable Hill coefficient. On the basis of an analysis of the kinetics of thermal inactivation and the influence of the temperature of preincubation of the enzyme on its activity, a greater thermal stability of the purified preparation was established in comparison with the unpurified preparation.