In 6 patients with Duchenne dystrophy, electron microscopical studies of concanavalin A (Con A) binding sites using the peroxidase labeling technique revealed a population of muscle fibers in which the reaction at the cell surface was irregular and patchy in areas larger than 0.5 mu. In 5 control subjects, such focal alterations were not observed and the reaction appeared continuous and regular. The findings indicate focal abnormalities at the muscle cell surface of Con A receptors in Duchenne dystrophy.