In order to observe the influences of spleen on the leucopoietic function of bone marrow the author performed the bone-marrow tissue culture (by cover-slip method) of rabbits, and obtained the following results. 1. In the observations on the bone-marrow tissue culture of the splenectomized rabbits conducted from time to time, the relative growth rate, the cell density index and the wandering velocity of pseudoeosinophils all show a transient acceleration with their peak around 4 to 7 days after the operation, while on the contrary the carbon particle phagocytic ability of pseudoeosinophils shows a transient fall. However, all these values return to respective levels of the control, normal rabbits without splenectomy, about 3 weeks after the operstion. 2. In the bone-marrow tissue culture of the rabbits transplanted with homoioplastic spleen conducted from time to time, the relative growth rate, the cell density index and the wandering velocity of pseudoeosinophils all show a transient fall around 5 to 10 days after the operation, but no marked change can be observed in the carbon particle phagocytic ability of pseudoeosinophils. However, all these values resurn approximately to those of the control, rabbits transplanted with normal kidney, about 30 days after the operation. 3. In the bone-marrow tissue culture of normal rabbits with addition of Ringer's solution of normal rabbit spleen extract, the extract at a higer concentration has a suppressing effect on the relative growth rate, the wandering velocity and the carbon particle phagocytic ability of pseudoeosinophils, while the extract at a lower concentration mostly gives a slightly accelerating effect. 4. From these results it seems that the spleen of normal rabbits with aid of some fluid agent adjusts the production of the leucocyte series of the bone marrow either suppressingly or acceleratingly. However, from the physiological aspect the suppressing action seems to be slightly superior and hence the over-production of the granulocyte series of the bone marrow as well as the incomplete maturation of granulocytes seem to be controled.