Acidic eluates from two rejected kidney allografts contained antibodies reacting only with the endothelial cells and the monocytes of the specific kidney donors. No reaction was observed with the B-cells or the T-cells of these donors. When these eluates were tested against the leukocytes of healthy blood donors that were typed for HLA-A, -B, -C and DR, positive reactions were observed with the monocyte but not with the lymphocyte fraction. The first eluate reacted with 6.4% of the donors and the second eluate with 38.3%. No correlation with any of the HLA-A, -B, -C or DR antigens was found. If a more extended study proves that the endothelial-monocyte antigens do play an important role in kidney transplantation, matching of donors and recipients for these antigens and cross-matching recipient serum with donor monocytes may become a necessity in the future.