Transplantable thyroid tumors were induced by grafting thyroids of mice with an autonomous but functional thyrotropic tumor. The latter provided the necessary stimulus of sustained high levels of thyrotropin. These thyroid tumors remained fully dependent after four consecutive animal passages, i.e., they failed to grow in the absence of a thyrotropic tumor. Although remaining fully dependent in the course of four passages, the tumors gradually assumed the histological appearances associated with malignant growth as manifested by cellular pleomorphism, loss of polarity, and frequency of mitotic figures. The tumors appeared to be locally invasive, but there was no evidence of distant metastases. The latency of the tumors was long (8-10 months) when the thyroid and thyrotropic tumors were grafted simultaneously. It could be reduced greatly by grafting the thyrotropic tumor much before the thyroid tumor.