Muscles from Becker muscular dystrophy (BMD) and Duchenne muscular dystrophy (DMD) patients were analysed using monoclonal and polyclonal antibodies raised against different regions of the dystrophin molecule. On blot, two of the antibodies detected a protein of Mr 400K in muscle extracts from all patients, including a BMD patient with a deletion which spanned more than 40% of the central rod domain of the Xp21 encoded dystrophin. Immunocytochemical labelling of tissue sections from the same patients showed that the same two antibodies labelled a protein at the surface membrane of smooth muscle fibers in blood vessels of both BMD and DMD muscles. Thus we have demonstrated a 400K blood vessel-associated protein, which is immunologically homologous with dystrophin, for at least two epitopes from the carboxy terminal and the central rod domains must be encoded by another gene than the dystrophin gene.