A lack of the cytoskeletal protein dystrophin causes muscle fiber necrosis in Duchenne/Becker muscular dystrophies (DMD/BMD) and in murine X-linked muscular dystrophy (MDX). However, no overt disease symptoms are observed in dystrophin-less cultured myotubes, and the biological function of dystrophin in normal muscle cells is still unknown. In this work, we have extended our studies on a model system, using hypoosmotic shock to determine stress resistance of muscle cells. In frozen sections of control human and mouse myotubes, dystrophin was shown to be localized at the cell periphery as in mature muscle fibers. Dystrophin-less DMD and MDX myotubes were more susceptible to hypoosmotic shock than controls, as monitored by the uptake of external horseradish peroxidase and release of the soluble enzymes creatinine kinase or pyruvate kinase and of radiolabelled proteins. Control experiments indicated that this difference is not due to differences in metabolism or ion fluxes. Treatment with cytochalasin D drastically increased the shock sensitivity of myotubes and abolished the difference between dystrophin-less and control cells. These results lend further support to the suggested stabilizing role of dystrophin in the context of the membrane-cytoskeletal complex.