Korean radish peroxidase was inactivated by addition of cystine-specific reagents such as 2-mercaptoethanol or dithiothreitol (DTT) known as Cleland`s reagent. The inactivation of the enzyme by dithiothreitol was affected by concentration of the reagent, temperature of the reaction, and time of incubation, pH of buffer used, and presence of protecting compounds. The inactivation of the enzyme by dithiothreitol was protected by adding substrates, hydrogen peroxide or aminoantipyrine/phenol. Therefore, the inactivation of the enzyme by Cleland`s reagent seems due to the breakage of disulfide bonds in the enzyme.