Human eosinophil peroxidase is a cationic protein with a higher content of arginine, the enzyme being poorly soluble in water. The purified enzyme is able to carry out the peroxidative chlorination of monochlorodimedon. Like myeloperoxidase the position of the pH optimum of this reaction depends on the ration of the concentrations of chloride and H2O2. Compared to myeloperoxidase the pH optimum is shifted by 0.8 pH unit to more acid pH values. The physiological consequences of the properties of the eosinophil peroxidase are discussed.