The monoclonal antibody to N-epsilon-(hexanonyl) lysine (HEL), a novel adduct formed by the reaction of linoleic acid hydroperoxide and lysine, has been prepared and characterized. The obtained antibody specifically recognized the HEL moiety, Using the monoclonal antibody, we evaluated the protective effects of feeding eriocitrin, which is one of flavonoids in lemon fruit, on oxidative modification induced by exercise in rats. The supplementation of eriocitrin significantly suppressed the increase in HEL in the skeletal muscle by exercise. The result suggests that the determination of HEL may be a good method for evaluation of the protective effect of beneficial food factors against oxidative stress.