Hematological and histological investigations were performed in mice with myelogenous type of leukemoid reaction induced by transplantation of MY sarcom. Thereafter, tissue culture of bone marrow and spleen were carried out. The following results were obtained. 1) Tissue culture of bone marrow. The growth patterns in bone marrow culture were always normal and did not display the leukemic type throughout the entire course of leukemoid reaction. The growth ratio of the pattern increased as leukemoid reaction progressed, while the cell density was fairly constant through the course of leukemoid reaction. Wandering velocity of the neutrophils was low in the early and intermediate stages and did not show difference from the controls in the advanced stage. Cytological observations by phase contrast microscopy, vital staining and fluorescence microscopy did not revealed any difference from normal marrow cells, nor atypism which was usually seen in leukemic cells. 2) Tissue culture of spleen. The normal pattern was always observed as seen in bone marrow tissue culture through the entire course of leukemoid reaction. The growth ratio increased as the reaction progressed. However, the cell density did not show difference from the controls. In leukemoid reaction, the lymphocytic cells were gradtally replaced by the neutrophilic cells. Therefore, 70 to 80 % of the cells in the growth zone of the splenic tissue culture were occupied by neutrophilic cells. In fluorescence tissue culture, the growth zone seen in leukemoid reaction displayed diffuse reddish orange color as a result of a large number of neutrophilic cells present, while the zone observed in controls showed diffuse yellowish green color owing to abundant lymphocytes. These findings were very helpful for differentiation of the splenic tissue in leukemoid reaction from the controls. The findings described above seemed to demonstrate the development of extramedullary hematopoiesis in the spleen.