The mdx mouse, although a genetic and biochemical homologue of human Duchenne muscular dystrophy (DMD), presents a comparatively mild histopathological and clinical phenotype. These differences are partially attributable to the greater efficacy of regeneration in the mdx mouse than in DMD muscle. To lessen this disparity, we have used a single dose of X-irradiation (16 Gy) to inhibit regeneration in one leg of mdx mice. The result is an almost complete block of muscle fiber regeneration leading to progressive loss of muscle fibers and their replacement by loose connective tissue. Surviving fibers are mainly peripherally nucleated and, surprisingly, of large diameter. Thus, X-irradiation converts mdx muscle to a model system in which the degenerative process can be studied in isolation from the complicating effect of myofiber regeneration. This system should be of use for testing methods of a leviating the myofiber degeneration which is common to mdx and DMD.