An experimental model of carditis has been produced in the rhesus monkey by giving 12 weekly injections of streptococcal membrane antigen. Carditis was produced in as short a period as 14 weeks. There was evidence of myocarditis, endocarditis, and, in two animals, myocardial granuloma formation. No valvular lesions were seen. Measurement of immune responses showed that heart cross-reactive antibodies started appearing in the circulation after the second injection. By the sixth injection, there was evidence of complement consumption and appearance of circulating immune complexes. Antibody-dependent cell cytotoxicity started operating after the second injection, and by the sixth injection, peripheral lymphocytes had acquired hypersensitivity to membrane antigen. It is concluded that some of these immunologic responses might have played a role in the genesis of carditis.