By means of bone-marrow tissue culture the author investigated the influences of ACTH and adreno-cortico-steroid hormones on the function of megakaryocytes, and obtained the following results: 1. The pituitary body and the megakaryocyte function In the case of repeated administration of ACTH to normal guinea pigs the function of megakaryocytes is accelerated, whereas in the case of the rats whose pituitary body is removed, it has been found that the megakaryocyte function is decreased. When ACTH, cortisone, testosterone or the powder of the thyroid gland is added directly to the bone-marrow tissue culture of the rat removed of the pituitary body, the function of megakaryocytes is restored most effectively by ACTH, followed by cortisone and the power of the thyroid gland, in that order, but testosterone hardly affects the function. On the other hand, in the direct addition of ACTH to the bone-marrow tissue culture of normal person, the megakaryocyte function is also increased, proving that ACTH acts directly on the megakaryocyte function without mediation of the adrenal cortex. 2. The adrenal cortex and the megakaryocyte function: In the case of repeated administration of cortisone to normal guinea pigs, the megakaryocyte function is accelerated, while in the guinea pigs whose adrenal gland is removed, the megakaryocyte function is diminished. Furthermore, when cortisone is added directly to the bone-marrow tissue culture of normal person, the function of megakaryocytes is accelerated, and likewise the direct addition of predonisolone to the bone-marrow tissue culture of normal person enhances the megakaryocyte function.