Investigations of antigens from three clinically well perfused human kidneys have been performed using xeno-antibodies of rabbit. The kidney preparations contained immunogenic amounts of normal human serum proteins. After absorption with autologous hepar tissue there were no signs in the absorbed anti-renal sera of remaining antibodies to serum protein if tested in double diffusion in agar gel. If tested against these hepar absorbed anti-renal sera, the saline extracted renal specific antigens were found to be thermolabile, trypsinstable and pepsinstable. A kidney cortex extract obtained after heating at pH 2.0 revealed a further antigen cross-reacting with the thermolabile kidney specific antigens. No indication of individual specificity could be found in double diffusion in gel analysis. The different anti-renal sera were equally cytotoxic to different human lymphocyte suspensions. The hepar absorbed anti-renal sera showed weak or non-lymphocytotoxic reactions to different human lymphocyte suspensions.