By means of bone-marrow tissue culture the author pursued the influences of thyroid hormones on the function of megakaryocytes, and obtained the following results: 1. By repeated administration of thyroxine to normal guinea pigs the megakaryocyte function is increased, and the megakaryocyte function in the patient with Basedow's disease has been found to be accelerated. 2. The removal of thyroid gland from normol guinea pigs diminishes the function of megakaryocytes, and on the other hand, the megakaryocyte function in the patient with myxedema has been diminished. 3. When the powder of the thyroid gland is added directly to the bone-marrow tissue culture of normal person, the megakaryocyte function is accelerated.