Surface radioiodination of lymphocytes by the lactoperoxidase procedure has permitted demonstration of an assortment of different antibodies to lymphocytes in the sera of patients with systemic lupus erythematosus. One type of antibody proved of special interest because it appeared to be associated with inhibition of the responder cells in the mixed leukocyte culture reactions. This reacted with an antigen on T cells and thymocytes which on sodium dodecyl sulfate acrylamide gel electrophoresis showed a molecular weight of approximately 15,000 daltons. The possible relation of this antigen to T cell receptors and products of immune response genes is discussed.