1. The mean survival of rats and mice bearing transplanted lymphomas was significantly prolonged by the administration of propylthiouracil or by thyroidectomy; this effect was inhibited by the administration of thyroxine.

  2. In rats rendered hypothyroid by propylthiouracil, the lymphomatous infiltrations were less extensive than in the euthyroid controls.

  3. The administration of thyroxine to euthyroid hosts hastened their death from transplanted leukemia.

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Work performed at Oak Ridge under Contract No. W-7405-eng-26 for the Atomic Energy Commission.

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