In the previous papers we reported on the isolation of piricularin and picolinic acid, the toxins of Piricularia orizae, from the culture broth of the fungus and the rice plant, severly infected with blast disease, and then on the biochemical observations of the effects of these toxins to the rice plant. To determine whether or not piricularin takes part in the blast fungal infection, we investigated the effect of a piricularin-detoxifying substance on the resistance of rice tissue to this infection. As a piricularin-detoxifying substance we chose ferulic acid (4-hydroxy-3-methoxycinnamic acid) which is a minor ingredient of polyphenols of rice plant, since it is easy to obtain by synthesis and it has not an anti-blastfungal activity. The experimental results demonstrated that the application of ferulic acid to the rice plant causes an increase in the tissue-resistance to the blast-fungal infection. The following facts support the view that the increase in the resistance to the infection of rice plants treated with ferulic acid depends upon the piricularin-detoxification: (a) Neither ferulic acid itself nor its oxidized product by oxidase exhibits any anti-blastfungal activity at 1/5, 000 dilution with the pH level of rice juice. (b) Not only ferulic acid but its oxidized product by oxidase also has a piricularin-detoxifying ability. (c) The juice pressed from the rice plant applied with ferulic acid also possesses a piricularin-detoxifying ability, exhibiting no inhibitory effect on the germination and growth of blast fungal spores. (d) The application of ferulic acid causes a respiration-rise of the rice plant, but this respiration-rise does not couple with oxidative phosphorylation which is to be related to the resistant reaction of the host tissue against the infection. The above mentioned observations lead to the conclusion that piricularin would play an important role in the blast-fungal infection together with picolinic acid suppressing the resistant reaction, i.e. the hypersensitive reaction, of the host tissue to the infection.