Periodic comparisons were made of sera from two groups of patients, ten who rejected their renal transplants within a year after transplantation and ten who successfully maintained their transplanted kidneys for five years or more. What appeared to be anti-kidney cytotoxic antibodies were found in much higher levels in the sera of those patients with the short-lived transplants, the difference in titer levels between the two groups being significant at the .0001 levels. This antibody showed no correlation with either the presence or the absence of lymphocytotoxic antibodies, nor did it appear to have any relationship to the HL-A antigens or the previous renal disease of the recipient patient. While it was cytotoxic to tissue cultures of cells obtained from random human kidneys, human kidney tumor cells (Wilms), and human embryonic kidneys, this antibody did not react with non-renal human tissues (lung, spleen, deltoid muscle, foreskin). It did not react with nonhuman (simian) kidney tissue culture cells. The findings suggest the appearance of an organ-specific, cytotoxic anti-kidney antibody in patients undergoing renal homograft rejection.