By means of bone-marrow tissue culture, the author studied the megakaryocyte count and the function of megakaryocytes in patients with various blood diseases, and obtained the following results: 1. In purpura simplex no abnormality could be observed in the megakaryocyte count or in the function of megakaryocytes. 2. In the cases of Banti's disease the megakaryocyte count was markedly increased, showing the increase of 2-5 times the normal count; and the megakaryocyte function was strikingly decreased, indicating the degeneration at an early stage. Even in these cases after splenectomy both the function and the number of megakaryocytes were found back to the normal level. 3. In hypoplastic anemia both the megakaryoccyte count and the function were found markedly decreased. 4. In the cases of symptomatic thrombocytosis, the megakaryocyte count increased slightly, while the function was greatly accelerated. 5. In pernicious anemia the megakaryocyte count showed an intermediate degree of decrease and likewise the megakaryocyte function was lowered. However, when vitamin B(12) had been injected, both the number and function of megakaryocytes returned normal. 6. The cases of agranulocytosis showed the normal count of megaryocytes and normal or slightly-accelerated function. 7. In the cases of essential hypochromic anemia the megakaryocyte count was slightly increased but the megakaryocyte function was normal. 8. In the cases of chronic hemorrhagic anemia both the megakaryocyte count and function remained at the normal level.