The effects of the phyotoxin from the fungal pathogen Rhizoctonia solani, causing sheath blight on the expression of defense-related proteins of rice were investigated. The toxin inactivated by chemical treatment and by the toxin-inactivating enzyme α-glucosidase produced by Trichoderma viride was used in the study along with the active toxin. Toxin inactivated by T. viride α-glucosidase and sodium periodate caused significantly less damage and electrolyte leakage to test plants. The active toxin and the pathogen induced chitinase and ß-1,3-glucanase synthesis in rice plants, while the inactivated toxin did not have any effect on the expression of these pathogenesis-related proteins. The toxin was found to suppress the peroxidase activity 72 h after inoculation and the inactivated toxin restored the activity as that of untreated plants. There was no remarkable change in phenylalanine ammonia lyase activity in rice sheath treated with both the forms of the toxin. © 2001, Taylor & Francis Group, LLC.