Cultured muscle cells from patients with Duchenne muscular dystrophy differed from cells of normal individuals and of patients with other muscle diseases. In Duchenne cells, basal activity of adenyl cyclase of myotubes was higher and was not stimulated significantly by epinephrine or isoproterenol, as it was in fused control cells, and the response to fluoride was less. The genetic defect in this disease may be an abnormality of the surface membrane of muscle.