Electron paramagnetic resonance spectra of leaves of rice cultivars Red rice, Labelle, Leah and Lemont with different level of resistance to the red toxin cercosporin, isolated from the fungus Cercospora oryzae Miyake, were assayed. The punctured leaves were treated with 20 μl, 100 μM cercosporin in 10% acetone and with 10% acetone alone in the same quantity per 2 cm2leaf area. The controls were punctured leaves only. An increase of the free radical production was recorded by electron paramagnetic resonance measurements after treatment with acetone and with cercosporin. We were found that the relative quantity of the free radicals correlated significantly well with the level of resistance of studied rice cultivars to the toxin.