SUMMARY Eluates were prepared from 27 transplanted kidneys from human renal allograft recipients. Seven of these eluates had alloantibodies demonstrable by at least one of the following techniques: cytotoxicity, leukocyte agglutination, and mixed agglutination. There was no apparent correlation between the histology or rejection category and the detection of antibodies in the eluate. The antibody activity of the eluates differed markedly from that present in serum samples taken at the time the transplanted kidney was removed. One of the eluates contained cytotoxic antibodies specific for HL-A12. Good antibody activity could be recovered from a kidney that was rejected in a hyperacute manner. Sephadex G-200 gel filtration studies of two of the eluates indicated that the antibody activity was associated with IgG immunoglobulins.