The localization in cardiac muscle and the biochemical properties of fibronectin, filamin, and vinculin were studied. Fibronectin was localized between cardiomyocytes. Filamin was identified in the Z-line region of sarcomers and in the intercalated disks of heart muscle. Vinculin was found to be present in intercalated disks and near the plasma membrane at the cell periphery between external myofibrils and sarcolemma. It was suggested that fibronectin, filamin, and vinculin play an important role in intercellular and intracellular linkages in cardiac muscle.