A potential treatment for Duchenne muscular dystrophy (DMD) is injection of normal myoblasts into dystrophic muscles to induce formation of muscle fibers. To develop this therapy it is important to identify the injected myoblasts and the muscle fibers that they form in the host muscles. Fluorescent latex microspheres (FLMs) were used for this purpose in this study. Normal myoblasts were labeled with FLMs and injected into dystrophin-deficient (mdx) mice. The FLMs clearly indicated the location of injected myoblasts in the host muscle. Muscle fibers containing dystrophin were localized by immunofluorescence and immunoperoxidase. They were observed in clusters near the myoblasts labeled with FLMs. FLMs were also observed in some of these dystrophin-positive fibers in each cryostat section. These results indicate that: labeling myoblasts with FLMs can be used to trace the injected myoblasts in the muscle and to identify the muscle fibers that they formed; injected myoblasts remain near the injected site and do not migrate very far; most of the dystrophin-positive muscle fibers around the injected myoblasts result from fusion of the injected myoblasts; and the low percentage of dystrophin-positive muscle fibers is likely related to limited diffusion and lack of fusion of many injected myoblasts.