A treatment has been developed to alleviate muscle weakness in murine dystrophy. Cultured myoblasts from genetically normal mouse embryos were injected into the right soleus of 20-day-old normal or dystrophic mice. Hosts and donors were immunocompatible but exhibited different genotype markers. Donor cells produced GPI-1CC. Host cells produced GPI-1BB. When compared with contralateral controls 6 months postoperatively, test dystrophic solei exhibited greater cross-sectional area, total fiber number, wet weight, and twitch and tetanus tensions. They contained more normal-appearing and less abnormal-appearing fibers. Their mean fiber resting potential was similar to that of normal controls. Presence of GPI-1CC with or without the hybrid isozyme GPI-IBC in these mucles implied the survival and development of donor myoblasts into normal myofibers, and fusion of normal myoblasts with dystrophic satellite cells to form genetically mosaic myofibers. Injection of fibroblasts instead of myoblasts caused detrimental effects.