The value of analysing dystrophin on Western blots of skeletal muscle for the differential diagnosis of Xp21 muscular dystrophies is now fairly well established. Here we describe a sensitive system based on monoclonal antibodies to dystrophin. The specificity of the antibodies was established and experiments were undertaken to identify the source of dystrophin-related protein bands which were detected on blots of normal skeletal muscle. These investigations formed a necessary preliminary study to the application of the assay to samples of muscle obtained at biopsy from patients with Duchenne and Becker muscular dystrophy.