We have identified the gene defect and breakpoint of δ-sarcoglycan (SG) in cardiomyopathic (CM) BIO 14.6 hamsters, which are a representative model of human cardiomyopathy. The aim of the present study is to explore a gene therapy using gene transfection to in vivo myocardial cells that we have previously established. Liposomes containing a reporter gene (β-galactosidase, β-Gal) with or without the δ-SG gene were coated with UV-inactivated hemagglutinating virus of Japan and injected into the left ventricular free wall in anesthetized open-chest CM hamsters. We prepared site-directed antibody to the extracellular domain of the δ-SG protein, the amino acid sequence of which was deduced from the cloned cDNA. Affinity-purified antibody demonstrated a single band after Western blotting in the normal Syrian hamster heart but no band in the CM heart. On day 7 after the transfection, hearts were removed and fixed. The adjacent cryosections were immunostained with antibody to β-Gal or δ-SG using the ABC procedure. The δ-SG gene product was clearly expressed in both sarcolemma and cytoplasm of cardiomyocytes of CM hamsters, whereas the β-Gal protein was exclusively coexpressed in the cytosol, showing the efficient cotransfection of both β-Gal and δ-SG genes and the slow protein traffic to the sarcolemma after biosynthesis of δ-SG in cytoplasm. We have succeeded in an efficient expression of the δ-SG gene in living CM hamster hearts with deleted δ-SG gene, suggesting that the newly developed in vivo gene transfection might be promising for the rescue of targeted CM cells.