As a link in the series of studies on the leukemoid reaction, mainly the bone marrow tissue cultures were carried out together with the studies of clinical symptoms on the 162 leukemia patients admitted to our Clinic for the period of from January, 1954 to December, 1960. The following conclusions were drawn. Fifty-five cases (34.0%) out of the 162 patients of leukemia revealed leukemoid reaction. The incidence of the leukemoid reaction proved to be high in the patients with acute lymphocytic leukemia and chronic myelogenous leukemia, followed by monocytic leukemia, and some cases in acute myelogenous leukemia. The severity of the leukemoid reaction was relatively mild in the majority of cases, but some show a marked reaction making the type difficult to discern. In such instances, it was possible to identify a composing cell series by the tissue culture method. As for the cell constituents in the blood attributed to leukemoid reaction in acute myelogenous leukemia, it was either of the neutrophil, the eosinophil or the monocyte series. In Xacute lymphocytic and monocytic leukemias it was mostly of the neutrophil series and, in some cases, either of the eosinophil or the basophil series. In chronic myelogenous leukemia it consisted of both the eosinophil and basophil series. As can be seen from these findings, in leukemias, the leukemoid reaction often associated with other white cell series than their own leukemia cell types. Therefore, the diagnosis of a type of leukemoid reaction should be made carefully.