We have demonstrated expression of a 6.3 kb Becker muscular dystrophy (BMD) human dystrophin cDNA following retroviral-mediated transduction of cultured myoblasts from the dystrophin-deficient mdx mouse. The truncated dystrophin protein was localised to the sarcolemma of differentiated myotubes by antibodies against the C-terminus of the molecule, and produced an identical immunostaining pattern to that observed in control myotubes expressing normal endogenous dystrophin. These results indicate that retroviral-mediated gene transfer may be useful for experimental in vivo studies on the complementation of dystrophin gene mutations.