Absorption of the kidney-localizing antibody from an anti-rat-kidney serum with several fractions of kidney homogenate demonstrated that the kidney-tissue antigen which combines with this antibody in vitro is contained only in the readily sedimentable fraction of the kidney homogenate. This antigen, furthermore, is not extractable from such sediment by saline solutions of even high concentration, or by acetone and ether, and it is remarkably heat stable. Liver homogenate, likewise, can absorb the kidney-localizing antibody, although to a lesser extent than can kidney homogenate. Elution of I131 from homogenates of kidney containing in vivo localized radio-iodinated anti-kidney antibody is accomplished with weakly alkaline solutions (about pH 11), but is not achieved with saline solutions or with weakly acidic solutions. The inability of iodide ion, (non-radioactive) to elute I131 shows that the iodine label does not exchange with iodide ion. The parallel behavior of the kidney-localizing antibody and the nephrotoxic principle of anti-kidney sera indicates the probable identity of these substances. The results obtained, therefore, support the hypothesis that the nephritis produced by anti-kidney sera is associated with the specific interaction of an anti-kidney antibody with an antigen in kidney tissue.