Polyclonal antibodies against the carboxy-terminal portion of dystrophin-related protein (DRP), the putative autosomal gene product which shares sequence homology with dystrophin, show the clear expression of DRP in mouse fetal muscle and in cultured human muscle cells, but not in mature mouse or human muscle. DRP has the same molecular mass as X-linked dystrophin and is recovered from the membrane fraction, but is associated with membranes more loosely than dystrophin.