We synthesized a peptide designated R8 (amino acid residues 1157-1201) based on the primary structure presumed from the nucleotide sequence of the cDNA clone from the gene for Duchenne muscular dystrophy. Antibody to the synthetic R8 generated by immunization of rabbits was tested on human and mouse skeletal muscle by Western blotting analysis. The antibody reacted with a component of the 400K dystrophin of normal human and mouse skeletal muscles, but not with components of the muscles of Duchenne muscular dystrophy patients and mdx mice. Thus we established that this peptide sequence is in fact missing in the protein product 'dystrophin' encoded by the DMD gene. The antibody may prove useful for the diagnosis of the Duchenne types of muscular dystrophy.