(1) The properties of the 5 cardiac antigens in human heart which cross-react with rabbit anti-rabbit heart auto-antibodies were studied, and compared to the rabbit heart auto-antigens analyzed previously. They were found to be similar in many respects. All were rendered undetectable by proteinases, but were unaffected by ribonuclease or desoxyribonuclease; they were salted out by exposure to similar concentrations of ammonium sulfate or potassium phosphate; they were variably labile to heat or pH extremes; and their elution characteristics from hydroxyl apatite and Sephadex G-100 were similar. (2) Further evidence was obtained that the human cardiac proteins detectable with rabbit anti-rabbit heart auto-antibodies were found only in human heart, and were not present in human skeletal muscle, kidney, pancreas, liver or plasma. (3) Passive hemagglutination assay correlated with immunodiffusion tests in terms of organ specificity and auto-antibody titers. (4) The precipitin systems formed between rabbit anti-rabbit heart auto-antibodies and human heart extracts failed to show esterase, catalase, glucosaminidase, glucuronidase, cysteine-amino-peptidase or phosphatase activities, as had been found earlier for the rabbit heart auto-immune system. Cardiolipin was found to be unrelated to any of the above human cardiac auto-antigens. (5) Urea, with or without dithiothreitol, incactivated only 2 of the human cardiac auto-antigens.