Resistance-inducing compounds, 4-hydroxybenzoic hydrazide (4HBHZ), salicylic hydrazide (SHZ) and 2-furoic acid (2FA) inhibited tomato cell wall-bound peroxidase and horse radish peroxidase (HRP) activities. However, salicylic acid (SA), a non-inducer of resistance against Fusarium wilt did not inhibit peroxidase activity. HRP was allosterically regulated by the substrate. These resistance-inducing compounds acted as inhibitory effectors of HRP. On the contrary, 4-coumaric acid (4CA) and salicylhy-droxamic acid (SHAM) acted as stimulatory effectors. Among these compounds, 4HBHZ and SHZ were bound to compound III, an inactive intermediate of peroxidase. Furthermore, 4HBHZ weakly activated compound III and acted as a cofactor of the NADH-dependent peroxidase reaction as did 4CA and SHAM.