An erythrocyte autoantigen has been identified by means of monoclonal autoantibodies raised by immunizing mice with rat red blood cells (RBC). The autoantibodies reacted with intact rat and mouse RBC as judged by a cellular radioimmunoassay, and with a 52K band on western blots of rat and mouse RBC. They did not react with intact sheep RBC or blotted sheep erythrocyte membranes. Although anti-rat erythrocyte antibodies did react with bands in the molecular weight region of 50-55 K (on blots of rat erythrocyte membranes), these bands were susceptible to neuraminidase digestion, thus distinguishing them from the 52 K band recognized by monoclonal autoantibodies. The implications of the above results for the known autoantibody specificity of suppression is discussed, and it is suggested that they favour the existence of autoantigen-specific suppressor cells.