A possible treatment for Duchenne muscular dystrophy is the injection of normal myoblasts into dystrophic muscles to induce the formation of new, healthy, and dystrophin-positive muscle fibers. To develop this therapy, it is important to identify the muscle fibers formed by the injected myoblasts in the host muscles. In this study, we used myoblasts from transgenic mice which have a gene expressing β-galactosidase under the control of the promoter of quail fast skeletal muscle troponin I. This transgene is expressed in myotubes and muscle fibers, but not in myoblasts. Twenty-eight days after myoblast transplantation in nude and in mdx mice, muscle fibers containing of β-galactosidase were identified by x-gal staining. In mdx mice, most of the β-galactosidase-positive muscle fibers resulting from the myoblast transplantation were also dystrophin positive. This technique could make it possible to follow the success of myoblast transplantation even in mice that are not depleted of dystrophin. © 1994 John Wiley & Sons, Inc.