Widespread deposits of bound gamma-globulin were observed in the ventricular muscle of a patient who died of acute rheumatic fever with severe cardiac failure. The possible occurrence of an antigen in group-A streptococci immuno-logically cross-reactive with heart tissue was investigated. Antisera prepared in rabbits against group-A streptococcal cell walls were found to exhibit an immunological reaction with human heart tissue by immunofluorescence and complement fixation. By use of special media free of mammalian tissue extracts, the role of medium constituents in the observed cross-reaction was excluded. In immunofluorescent studies, the reactant in human heart was distributed in cardiac myo-fibres and in smooth-muscle elements of arteries, arterioles, and endocardium. Serological reaction was abolished by absorption of antisera with streptococcal cell walls and acid extracts of whole streptococcal cells or cell walls, but not by cell protoplasm or protoplast membranes. Cross-reactive antigen in ammonium sulphate and chromatographic fractions of acid extracts was associated with M protein, but not with A carbohydrate. Immunization of rabbits with such active fractions elicited antibodies reactive with human heart. These observations support the hypothesis that in rheumatic fever an autoimmune reaction to a constituent of myofiber and smooth muscle may be induced by exposure to a cross-reactive antigen of group-A streptococci. The relation of the present observations to the varied manifestations of rheumatic fever remains to be determined.