By means of borne-marrow tissue culture, the author has investigated the wandering velocity, phagocytosis of carbon particles and neutral-red vital staining of neutrophils and monocytes in both patients and normal persons, and obtained the following results. 1. The wandering velocity of the neutrophils and monocytes in patients was lower than that in normal persons, and the span of cell-wandering in th former was shortened. 2. Phagocytosis of carbon particles of the neutrophils and monocytes in patients was decreased in comparison with that of the control. The time required for the phagocytosis of neutrophils in tissue culture to reach its peak is longer than that in the case of the control. The phagocytosis of the monocytes in patients gradually increased, making a similar curve as that of the control but a lower one, and both reached their peak in 24 hours. The peak of the former, however, was markedly lower. In addition, promonocytes occasionally showed very slight phagocytosis of carbon particles in the cases whose cell functions were not so markedly decreased. 3. As for the neutral-red vital staining the nutrophils and monocytes in patients were stained much more rapidly and more deeply but they fade very quickly as compared with those of the control. 4. From the above mentioned results, the author has confirmed the decrease in the cell functions of the bone marrow in monocytic leukemia.