With immunohistochemical staining using polyclonal antibodies against the carboxyl terminus of a dystrophin molecule, the surface membranes of fetal and denervated adult muscle fibers and the neuromuscular junctions in dystrophin deficient mdx mice showed positive immunoreactivity. The behavior of the positively stained protein was similar to that of the acetylcholine receptor. We conclude that this immunoreactive protein, probably a dystrophin-related protein, plays a role in the assembly of junctional proteins.