We examined normal and dystrophic human myotubes in cell culture for expression of dystrophin, the protein product of the Duchenne muscular dystrophy locus. Dystrophin levels in developing myotubes detected by Western blotting increased after 24 hours and reached maximum levels after 10 days in fusion medium. We did not detect dystrophin in myotubes cultured from Duchenne myoblasts (7 cases). Myotubes from a Becker muscular dystrophy patient's biopsy produced a lower molecular weight (approximately 408 kd) dystrophin, which was the same size in a whole muscle preparation from the same biopsy. This 408-kd dystrophin was the expected size for this Becker patient whose DNA was deleted for exons 45-48 of the Duchenne gene. This cell culture system will allow a detailed analysis of the effects of potential pharmacologic agents on steady-state dystrophin levels.