With a view to elucidate the actual characteristics of the bone marrow in hypoplastic anemia the author performed a series of bone-marrow tissue culture of the sternum in the patients with this disease and obtained the following results: 1. Many fat cells were observed in the explant of the bone marrow, and a marked decrease in the tissue growth area and cell density were observed as compared with those of the normal persons. Moreover, differences can be recognized According to different types (by prof Hiraki) of this disease: namely, Type I (the type of blood cell arrest) shows the least decrease in the tissue growth area and cell density; Type V (type of pan-myelophthisis) the greatest decrease; and Type II (the type of maturation arrest), Type III (the type of regeneration disturbance), and type IV (the mixed type) the intermediary decrease. 2. Looking at the neutrophil function in the bone marrow, both their wandering velocity and carbon-particle phagocytosis are decreased, and as for the vital staining of neutrophils they show the lower function type in which cells are stained early but fade quickly, too. In this instance as in the case of tissue growth a similar tendency can be observed according to different types of this disease. By the bone-marrow tissue culture the tissue growth and cell function in the bone marrow are lowered, showing significant differences according to the types classified by prof. Hiraki. Therefore, they offer the findings that are definitely valuable for an early diagnosis of this disease as well as for judging the effect of treatment and prognosis.