By means of the simple bone-marrow tissue culture with addition of sera from patients with various blood diseases the author observed the influences of these sera on the megakaryocyte function, and obtained the following results: 1. It has been found that the serum of hypoplastic anemia contained the factors that greatly diminish the megakaryocyte function. 2. In the serum of Banti's disease there are factors that lower the megakaryocyte function, bringing about the degeneration at an early stage. However, after splenectomy no such factor can be recognized in this disease. 3. In the serum of essential hypochromic anemia no factor has been found to affect the megakaryocyyte function. 4. In the serum of polycythemia vera there are factors which enhance the megakaryocyte function.