Lipoxygenase (LOX) and lipid hydroperoxide-decomposing activity (LHDA) markedly increased in the fifth leaves of rice (Oryza sativa cv Aichiasahi) after infection with the rice blast fungus, Magnaporthe grisea. The increases in the enzyme activities were significantly higher in response to infection with an incompatible strain (race 131) compared with infection with a compatible strain (race 007) of the fungus. Using ion-exchange chromatography, we isolated three LOX activities (leaf LOX-1, -2, -3) from both uninoculated and infected leaves. The activity of leaf LOX-3, in particular, increased in the incompatible race-infected leaves. The leaf LOX-3 had a pH optimum of 5.0 and produced preferentially 13-L-hydroperoxy-9,11 (Z,E)-octadecadienoic acid (13-HPODD) from linoleic acid. 13-HPODD and 13-L-hydroxy-9,11 (Z,E)-octadecadienoic acid, one of the reaction products from 13-HPODD by LHDA, were highly inhibitory to the germination of conidia of the fungus. The present study provides correlative evidence for important roles of LOX and LHDA in the resistance response of rice against the blast fungus.Rice (Oryza sativa) blast, caused by Magnaporthe grisea, is one of the most destructive rice diseases. Many studies have been concerned with resistance mechanisms of rice to the blast fungus, and, thus, several antifungal substances have been isolated from rice leaves (1, 4, 9-1 1, 14). However, the biosynthetic mechanisms of these substances in fungal-infected leaves have not been established. Therefore, it is not known whether the antifungal substances isolated from rice leaves are actually involved in the defense response of rice against the fungus.We recently found an activity that decomposed lipid hy-

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