Anatibodies directed against the plasma membrane of renal collecting ducts were found in the sera of two out of 101 renal allograft recipients. The sera also reacted with the epithelium of the renal pelvis, the ureter, the ductus deferens, the skin, the oesophagus, the trachea and the bile ducts of random individuals, indicating that the antibodies were directed against a widespread epithelial antigen. The antibody activity against these structures was removed by absorption of the sera with homogenized renal medullary tissue; absorption with plasma proteins, erythrocytes, spleen leucocytes or platelets did not influence the antibody titers. In the two patients with circulating antibodies, the graft function was unremarkable eight and fifteen months after transplantation, respectively. From this study we conclude that circulating epithelial antibodies directed against the collecting ducts of the graft are of little or no clinical importance.