Recently a tissue restricted antigen system, which is expressed on endothelial cells and monocytes (E-M antigens) but not lymphocytes, has been associated with kidney graft rejection. In screening sera from recipients of kidney, bone marrow or skin grafts for possible reactivity with endothelial cell antigens, we have found that all (13 of 13) endothelial reactive sera also reacted with monocytes, but that many (21 of 34) monocyte reactive sera did not react with endothelial cells. Additionally, one well-defined monoclonal antibody (M1/70), which was cytotoxic for human monocytes, neither stained renal endothelium nor was absorbed by renal endothelium when perfused through a human kidney. Thus, not all monocyte antigens appear to be expressed in high concentrations on renal vascular endothelium. This may explain why monocyte reactive antibodies do not always correlate with kidney graft rejection.