Antibody which had cytotoxic and agglutinating activity against donor lymphocytes appeared in the blood stream of primary renal allograft recipients usually within 48 h of the graft being finally rejected. Appearance of the antibody See PDF for Structure in the blood was associated with severe alterations in vascular permeability and this led to increases in the numbers of red cells and in the protein content of the lymph coming from the allograft. It was possible to elute cytotoxic and agglutinating antibody from renal allograft tissue, showing that this type of antibody was bound to graft antigens during the rejection process. The transfusion of whole serum or serum globulins obtained from sheep that had previously rejected allografts led to the destruction of recently installed renal grafts and the histological changes produced in these grafts and the alterations in the composition of the lymph coming from them were similar to those seen in the terminal stages of primary rejection. These findings have led us to the conclusion that in the sheep, at least the terminal stage of primary renal allograft rejection is mediated by humoral antibody.