In an attempt to investigate enzyme-catalyzing degradation of lipid peroxide in the living body, methyl linoleate hydroperoxide (MLHP) was tested as a substrate for peroxidatic reaction catalyzed by a commercial hepatocatalase (E. C. 1. 11. 1. 6.), Caperase. Autoxidized methyl linoleate was further fractionated on a cellulose-powder column with n-hexane then with methanol, and MLHP in the latter eluate was applied on the enzyme assay. The approximate reaction constant, k1, was calculated from the linear relationship of substrate concentration and the maximum velocity on the color development of the excess amount of guaiacol as a hydrogen donor. Although MLHP did not react with the intact enzyme at all, partial alkali-denaturation within the close pH range between 11.0 and 11.3 made it possible to catalyze the decomposition of both H2O2 and MLHP at approximately the same rate of the order of 102 sec. -1, mole-1, with higher value on MLHP than on H2O2, provided that one mole of hydroperoxide is contained per mole of MLHP.