Heart-reactive antibody (HRA) appears in the sera of experimental animals inoculated with group A streptococci as well as patients with acute rheumatic fever. Adsorption of either serum with group A streptococcal membranes will remove the HRA. Blocking experiments between these two types of HRAs have demonstrated that the antibodies are directed towards different antigenic determinants on either the same or different molecules. To isolate and purify the antigen from the group A streptococcus cross-reactive with sarcolemmal sheaths of cardiac myofibers, it became necessary to purify the HRA from rheumatic fever patients' sera. Isolated gamma globulin containing all of the HRA was adsorbed onto human sarcolemmal sheaths. The specific HRA was released by using potassium iodide. Over 99 percent of the purified HRA was shown to bind the sarcolemmal sheath whereas less than 1 percent of the antibody would bind nonspecifically to other material. Preparations of group A streptococcal membrane will bind HRA purified from the sera of acute rheumatic patients at levels of 97 percent or greater. The cross-reactive antigen solubilized by nonionic detergent was purified 120-fold by column chromatography. On sodium dodecyl sulfate polyacrylamide electrophoresis, the antigen was demonstrated to be composed of four polypeptides with mol wt of 32,000, 28,000, 26,000, and 22,000 daltons, respectively. Only proteolytic enzymes could destroy the antigenic determinant whereas glycosidases and lipases had no effect. The purified antigen blocked the binding of purified HRA to normal human heart sections.