In this paper we demonstrate that the vascular endothelium of a normal kidney and a syngeneic kidney graft binds only poorly syngeneic or allogeneic lymphocytes in an in vitro binding assay. On the contrary, the peritubular capillary endothelium of an allograft binds almost 7 times more lymphocytes than the peritubular capillary of a normal kidney. These findings suggest that the site of influx of inflammatory capillary white cells into the graft during rejection is the peritubular capillary endothelium.