A partially purified preparation of guinea pig eosinophil peroxidase was found to be bactericidal when combined with H2O2 and either iodide, bromide, chloride, or thiocyanate ions. The EPO-H2O2-halide bactericidal system had an acid pH optimum and was inhibited by the proteins albumin and gelatin and by the hemeprotein inhibitors azide, cyanide, and aminotriazole. When the EPO concentration of the reaction mixture was lowered, the bactericidal effect at pH 7.0 was lost first with chloride, then with bromide, and finally with iodide as the halide. Activity with physiologic concentrations of chloride was favored by a relatively high EPO level, a decrease in pH below neutrality and an absence of extraneous protein. These findings are discussed in relation to the potential role of the peroxidase system in the intracellular and extracellular toxic reactions of eosinophils.