An acid extraction procedure for the solubilization of cardiac antigens present in washed homogenates of myocardium is described. The organ specificity of antigenic components of acid extracts of human heart was studied by immunodiffusion and immunofluorescent techniques. Two antigens, restricted in organ distribution to heart and skeletal muscle, were demonstrated. As indicated by immunofluorescence, these antigens were both localized to sarcolemmal and subsarcolemmal structures of myocardial fibers. One of these antigens was found widely distributed among mammalian species while the other was found only in primate species. Both antigens were susceptible to digestion by trypsin and pepsin, and were found relatively thermolabile. Partial purification of these antigens was obtained by DEAE-cellulose chromatography and by Sephadex G 200 gel filtration. The relationship of heart antigens in such acid extracts to streptococcal cross-reactive antigens(s) is considered.