By means of extirpation experiments and from observations on the effects of administering thyroid gland preparations, it has been shown by Esser,1Mansfield,2Kishi3and others, that: (a) after thyroidectomy, dogs and rabbits show a decrease in red cells and hemoglobin amounting usually to about 30 per cent; (b) in animals with an experimentally produced anemia, regeneration is retarded by thyroidectomy; (c) administration of thyroid gland to normal animals may cause an increase of red blood cells up to 15 per cent. For the purpose of this paper it is not necessary to review these experiments in detail. They indicate that under controlled conditions it can be shown that the thyroid hormone exerts a certain influence on hematopoiesis. THE THYROID AND HEMATOPOIESIS Numerous clinical observations (Kocher,4Falta,5Bence and Engel,6Janney and Engel,7Hutton8) also provide unmistakable evidence that disturbances of the