AbstractRabbit anti‐rat anti‐human thyrotropin anti‐idiotypic antibodies have been raised. These antibodies were active at the thyrotropin (TSH) receptor, in that they inhibited125I‐labeled bovine TSH binding to thyroid plasma membranes, stimulated adenylate cyclase activity through a guanyl nucleotide‐dependent mechanism, augmented radioiodide transport into isolated porcine thyroid follicular cells and induced such cultured cells to organize into follicles. Aside from substantiating the expectation that anti‐hormone anti‐idiotypic antibodies may possess properties of the original hormone, this work raised the possibility that thyroid‐stimulating antibodies which cause the hyperthyroidism of Graves' disease may be anti‐TSH anti‐i