Rabbits were immunized with a hapten-protein conjugate and sera were collected for 189 days. The antihapten antibodies were purified by affinity chromatography, then the same animal that synthesized the antibody was reinjected with polymerized F(ab')₂ fragments of antihapten antibodies. Sera were collected after autoimmunization and tested by an indirect radioimmunoassay technique for reaction with [¹²⁵I]F(ab')₂ fragments of the original antihapten antibody. Results showed that each individual responded to its own F(ab')₂ and the antisera were specific for antihapten antibodies of that individual. Quantitative allotype assays established the immunoglobulin nature of the labeled test antigen. Inhibition assays showed that the reaction was specifically inhibitable with hapten. The relationship of this system with other idiotypic systems and the possible autoimmune implications of autoantiidiotypic antibodies are discussed.