A simple assay method for measuring myeloperoxidase (MPO) has been developed. MPO is found in polymorphonuclear leukocytes and is important as a bactericidal agent in the presence of H2O2 and halide ions. This improved assay method is based on work of Andrews and Krinsky using tetramethylbenzidine (TMB) a noncarcinogenic substrate. By assaying MPO under optimal conditions of TMB at 1.6 mM, H2O2 concentration of 0.3 mM, pH 5.4, and incubation temperature of 37 degrees C, sensitivity of MPO measurements increased eightfold in comparison with the original TMB method. A method has been established to determine absorbance at 655 nm of the reaction mixture by incubation for 3 min and then stopping the reaction by the addition of pH 3.0 buffer. An attempt was also made to raise the sensitivity by using 3,3'-dimethyoxybenzidine (DMB), a carcinogenic substrate. The improved TMB method was 34 times more sensitive than the DMB method.