By means of bone-marrow tissue culture the author studied the influences of various hormones such as ACTH, cortisone, and predonisolone on the megakaryocyte function in various blood diseases, and obtained the following results: 1. ACTH has bean found to act most effectively to restore the megakaryocyte function in idiopathic thrombocytopenic purpura, and cortisone and predonisolone follow in the effectiveness in that order. 2. Cortisone has been found to act most effectively to restore the megakaryocyte functon in aplastic anemia, followed by predonisolone and ACTH in the effectiveness in the descending order. 3. Predonisolone acts most effectively in restoring the megakaryocyte function in Banti's disease, follewed by ACTH and cortisone in the effectiveness in the order mentioned.