The blood picture resembling leukemia, known as leukemoid reaction, was observed in the neoplastic and non-neoplastic illnesses. Observation were made by means of routine laboratory as well as the tissue culture technics devised in our Department on patients with malignant neoplasmas who were admitted to our Clinic in the period between April, 1956 to June, 1961. Among them, nine cases developed leukemoid reactions. The results of study on these cases are briefly summerized as follows; 1. The leukemoid reactions seen in these patients (5 cases of gastric cancer, 2 mammary cancers, l pancreatic cancer and l lymphosarcomatosis) proved to be all of neutrophilic reactions. 2. Ahemorrhagic tendency was observed in 4 cases and their serum-iron level was slightly lower than normal. 3. The majority of these patients were of normochromic anemia, and the leukocyte counts varied, that is, one case showed a slight increase while another a decrease. Maturity of leucocytes consisted of myeloblasts in 3 cases and promyelocytes and myelocytes in almost all of the patients. This indicated that cells were in varying stages of maturation. Many erythroblasts were also recognized. 4. On examining the bone marrow, nucleated cells generally tended to decrease in the numbers, and was observed in any of these cases. 5. The growth patterns of the bone marrow using the tissue culture technic proved to be mostly of the normal type, and not a single case showed the leukemic pattern. Occasionally, a few tumor cells metastasizing into the bone marrow was also recognized. These findings indicated that the bone marrow tissue culture technic devised in our Clinic is an indispensable method for the differentiating the leukemoid reaction from leukemia.