Examining the functions of RES in patients with various blood diseases and in rabbits with experimentel anemia by Congo-red method advocated by Adler and Reimann and by the technique of carbon-particle phagocytosis in the subcutaneous tissue according to Sugiyama, the author studied the relationship between the hematopoiesis in bone marrow and the functions of the reticuloendothelial system (RES); and obtained the following results. 1. The Congo-red indices of various blood diseases are 71.9 in essential hypochromic anemia; 72.2 in hookworm anemia; 72.2 in Banti's disease; 72.7 in hypoplastic anemia; and 74.4 in leukemia, all showing about an equal degree of the diminution in the functions of RES. 2. As for the results of examinations of the RES functions in the rabbits with various experimental anemia whose hematopoietic functions of the bone marrow are disturbed by the injection of benzol, saponin, collargol, or irradiation of X-rays, not only the functions of the RES are disturbed but also the degree of anemia has been found to parallel to a certain extent with the change in the RES functions. Furthermore, in the blooddepleted anemia whose hematopoietic functions of the bone marrow are accelerated, the functions of RES have bean likewise accelerated. From these facts the functions of RES seem to be closely associated with the hematopoietic functions of the bone marrow. However, in the case of phenylhydradine anemia, despite the acceleration in the hematopoietic functions of the bone marrow, the functions of RES have been decreased. This seems to be due to the fact that RES is forced tc phagocytose and dispose of the decomposed substances of erythrocytes arising out of the hemolysis by phenylhydradine and therefore, the phagocytotic ability of RES for foreign substances is on the whole diminished. 3. In the examinations of the RES functions the change in the Congo-red index has generally paralleled with the change in the degree of the carbon-particle phagocytosis in the subcutaneous tissue.