Recent knowledge concerning the etiology of autoimmune disease makes the development of specifically cytotoxic immunotherapeutic reagents feasible. The spontaneously occurring autoimmune thyroiditis of OS chickens is an excellent animal model for Hashimoto's thyroiditis and ideal for testing these reagents. To develop these immunotherapetuic reagents, we will produce anti-idiotypic antibodies with specificity towards thyroid antigen-specific T and B lymphocytes and couple them to cytotoxic agents (A chain of ricin, daunomycin, or Alpha-Amanitin). We will also label one of the thyroid antigens involved in thyroiditis (thyroglobulin) with the same agents. These coupled molecules will be tested in vitro for specificity and cytotoxicity and then administered to yound OS chicks to prevent the development of thyroiditis. These reagents will also be administered to OS chicks after disease has begun, in an attempt at treatment. Successful completion of these aims will give major impetus to the development of similar reagents for the treatment of various autoimmune diseases afflicting man.