With some premonitions as regards the picture, of platelet formation, which has hitherto been used for the determination of the megakaryocyte functions, the author performed a few experiments in order to clarify this doubt; and obtained the following results: 1. When 3.8% citrate solution had been added at the time of preparation of stained bone-marrow specimens from normal persons or from patients with various blood diseases, an increase in the megakaryocyte count without platelet attachment was observed, showing no appreciable degree of the platelet attachment. Moreover, after the addition of the citrate solution, both in the case of a high degree and a low degree of the platelet attachment showed the same proportion of the attachment in respective case. 2. With the addition of normal human platelets in physiological saline solution at the time of preparation of stained bone-marrow specimens from hypoplastic anemia which never shows the so-called platelet formation, a high degree of platelet attachment was observed. From these findings it has been clarified that the so-called platelet formation is a picture showing platelets in the bone marrow drawn out simply adhering to the surface of megakaryocytes, and that this platelet formation can not be used as a criterion for the determination of the magakaryocyte functions.