Summary: Inoculation of rabbits with whole streptococcal cultures grown in beef heart medium was found to stimulate antibody reactive with normal rabbit heart sections, as determined by a modified immunofluorescent technique. The immunofluorescent reactivity of the serum was found to be correlated with complement-fixing antibodies to normal rabbit heart tissue suspension. This antibody response was not related to the streptococcus but to the beef heart medium in which the streptococci were grown. Immunization with beef heart medium alone or with homogenate of beef heart also elicited both types of antibody. Similar results were obtained following immunization of rabbits with homogenates of rat and human heart. These data give evidence of a cross-reacting myocardial antigen in the sarcoplasm of the several mammalian species tested.