The sera of patients with acute rheumatic fever, uncomplicated streptococcal infections and post-streptococcal glomerulonephritis contain a heart-reactive antibody which binds to the sarcolemmal membrane of myofibrils of heart muscle as well as to other mammalian muscle tissue. The sera of patients with acute rheumatic fever have about four times as much of this antibody as is found in sera of patients convalescent from uncomplicated streptococcal infections or acute post-streptococcal glomerulonephritis. This antibody is not present in a number of other diseases, including idiopathic myocardiopathies, lupus erythematosus, rheumatoid arthritis, multiple myeloma, and sarcoidosis. The titre of heart-reactive antibody declines slowly over a period of 2–3 years following acute rheumatic fever but reappears with subsequent rheumatic recrudescences. Following cardiotomy a heart-reactive antibody also is demonstrable in the serum but is differentiated from that antibody which follows acute rheumatic fever. While streptococcal-induced antibody is absorbed with streptococcal membrane, heart-reactive antibody following cardiotomy is absorbed only with cardiac tissue. The presence in the sera of patients with acute rheumatic fever of high titres of heart-reactive antibody, absorbable by streptococcal membrane, provides an additional laboratory tool for the differential diagnosis of rheumatic fever and may be of value in the long-term management of this disease.