The f ree radicals in soy bean leav es infected wi th 1, 7 races of Cercosporasojina Hara w ere measured by ESR. The results show ed that the free radical in leav es ofthe resistant v arieties at v arious time af ter ino culatio n wa s higher than tho se of thei rcont rast plants, and the free radical in the leav es of susceptible va rieties w as low er thanthose of thei r co nt rast plant s. Furthermo re, the chang es of f ree radical in leav es ofva rieties tested w ere somewhat correlated wi th the host resistance o r susceptibility toCercospora soj ina Hara.