Nineteen monoclonal antibodies which bind to native dystrophin in the plasma membrane of frozen muscle sections were obtained using a recombinant fusion protein as immunogen. On Western blots of normal mouse muscle extracts, the antibodies bind specifically to a 400,000 Mr protein which is absent from dystrophic mouse (mdx) muscle. At least four distinct epitopes have been identified by cleavage mapping methods. Although the fusion protein contained 25% of the human dystrophin sequence (Cys816-Asp1747; Mr 108,000), most of the monoclonal antibodies (15 out of 19) recognize a single fragment of Mr 27,500.