We determined several internal amino acid sequences of dystrophin-associated glycoprotein, A3a (one of the 43DAG doublets), of rabbit skeletal muscle. All the sequences of A3a determined were found in the C-terminal region of dystroglycan, which is the region assumed to be the cytoplasmic domain of 43DAG [Ibraghimov-Beskrovnaya et al. (1992) Nature 355, 696-702]. Therefore, A3a is identical with 43DAG. We raised an antibody (PA3a) against a synthetic polypeptide equivalent to one of the internal amino acid sequences of A3a. The antibody specifically reacted with A3a of rabbit skeletal muscle. PA3a, however, did not react with A3b, the other 43DAG doublet, suggesting that the 43DAG doublets are different proteins from each other. When the human control muscles were examined, PA3a immunohistochemically stained the cell surface membranes and exclusively reacted with a single protein similar to A3a in the SDS extracts. The protein was also detected in the SDS extract of the Duchenne muscular dystrophy (DMD) muscle devoid of dystrophin. When the muscle specimens from 30 DMD patients were immunohistochemically examined with the antibody, the cell surface membranes were consistently stained. Therefore, we conclude that the dystrophin-associated protein, A3a, is retained in DMD muscles.