Mdx mouse dystrophy is characterized by the absence in the muscle cytoplasmic membrane of a high molecular weight protein called dystrophin. A possible avenue for treatment of muscular dystrophies is to inject normal myoblasts in a dystrophic muscle to form hybrid muscle fibers. Hybrid myotubes were formed in vitro by the fusion of normal rat and dystrophic mouse (mdx) myoblasts. Staining with Hoechst dye 33258 permitted the clear distinction of mouse and rat nuclei. Immunostaining demonstrated that dystrophin was present over the entire membrane of all hydrid myotubes even when nuclei ratio normal/dystrophic was low.