To remove senescent red blood cells (RBCs) from the circulation, macrophages must distinguish them from mature RBCs. That is achieved by a specific recognition system. An antigen that develops on the surface of a senescing RBC is recognized and bound by the Fab region of an IgG autoantibody in the serum. Subsequently the Fc region of the autoantibody is recognized and bound by a macrophage, which proceeds to phagocytose the RBC. The antigenic molecule can be extracted from senescent but not young RBCs with Triton X-100 (ref. 4), although 10--30% as much antigen can be extracted from middle-aged as from senescent RBCs. I have now used IgG autoantibodies eluted from senescent RBCs to isolate and purify the IgG-binding antigen on senescent RBCs, and to detect the antigen on other somatic cells. The antigen is a congruent to 62,000-Mr protein which is present on stored platelets, lymphocytes and neutrophils, and on cultured human adult liver and embryonic kidney cells, as well as senescent RBCs.