Summary: Rabbits were immunized with preparations of rabbit heart with adjuvant. Only 4 of 20 animals responded with high titers of hemagglutination antibodies. No response was obtained without adjuvant. In the tanned cell hemagglutination test, antisera showed cross-reactions only with hearts of other species and the organ specificity was confirmed by inhibition and neutralization tests. Other serological methods failed to demonstrate this. Reactions of antisera to homologous heart are compared with those of antisera to heterologous hearts. The importance and possible mechanism of production of organ specific autoantibodies in animals and humans are discussed.