We have studied cytotoxicity of antisera of monkeys sensitized to streptococcal M protein fraction, which was prepared by the method of Lancefield et al., on cultured myocardial cells. These antisera exerted a significantly higher cytotoxic effect on the myocardial cells than normal monkey sera, and this cytotoxic effect seemed to be organ-specific. In the presence of normal monkey lymphocytes, the antisera had a cytotoxic effect on myocardial cells, but normal sera in the presence of normal monkey lymphocytes had no cytotoxic effects. These data suggest that M protein fraction of hemolytic streptococcus plays an important role in the pathogenesis of rheumatic carditis.