In the bone-marrow tissue culture of the patients with subacute bacterial endocarditis the author obtained the following results: 1. In the case of cover-slip culture, the relative growth rate is normal both before and and after the treatment of patients, but the cell-density index is normal or in some cases with a decrease in bone-marrow nucleated cells it is diminished. 2. In the case of the tissue culture in a fluid medium, the increasing rate of erythrocytes, reticulocytes, and hemoglobin is markedly low before the treatment, but it recovers with treatment. 3. The wandering velocity of neutrophils in the bone marrow has been found to be normal or slightly accelerated. 4. The carbon-particle phagocytotic ability of neutrophils in the bone marrow is markedly decreased before the treatment, but it recovers to normal when the patient is treated. 5. As for the neutral-red vital staining of bone-marrow neutrophils, these cells stain quickly and deeply, indicating a decrease in the functions, but this condition is restored to normal by treatment. In the present paper the author stated that the examinations of the bone marrow by tissue culture with emphasis on the carbon-particle phagocytosis served a quite significant purpose for clinical course observations.