The presence of Zinc in living organisms and its role as an essential nutrient for plants and animals has been recognised since 1869. However, its metabolism remains still widely unknown. In recent days, much attention has been given to the significance of Zinc metabolism in malignant tumors. The author tried to study the Zinc metabolism in leukemia and to find its clinical significance. The results of these studies are presented as follows. Materials and Methods. The subjects of the study consisted of 26 cases of acute myelogenous leukemia (AML), 8 cases of acute lymphocytic leukemia (ALL), 2 cases of monocytic leukemia (MOL), 9 cases of chronic myelogenous leukemia (CML), 2 cases of chronic lymphocytic leukemia (CLL), and 2 cases of erythrocytic leukemia (EL) to the total of 48 cases as well as 5 cases of malignant lymphoma were selected as related diseases, all admitted to the Second Clinic of Internal Medicine, Okayama University. For the control 13 males and 12 females to the total of 25 healthy adult persons were selected. For the purpose to study tthe correlations between th types of leukemia and the total Zinc score as well as Zinc score gram, and the mutual relationships between the Zinc score and clinical course after treatment we estimated the amounts of Zinc in peripheral blood granulocytes of leukemic patients by Mc Nary.s Method. In addition, the correlation was studied between the onset and the development of experimental leukemia. The results of these studies are presented as follows. Results and Conclusion 1) The total Zinc score in the healthy control group was 200.2±18.8, while the average value in leukemic patients was as low as 161±32.3 in most of the patients before treatment. 2) On looking at the total Zinc score according to types of diseases, CML showed a markedly low value of 126.0±12.3 in average, followed by a low value of 165.0±29.4 in AML, but the latter gave a considerable variation in the values. ALL and CLL did not show any significant difference from the control group. 3) On studying the relationship to treatment, there was a tendency of the low value recovering to normal level as the symptoms were improved by the treatment. 4) Looking at the Zinc score gram prepared by plotting the frequencies according to the grade of Zinc score the pattern differed somewhat according to the type of diseases, but on the whole there was observed a tendency to approach the normal pattern as the condition improved by treatment, while it tended to return to the pattern before treatment as the condition aggravated. Such tendencies were most marked in CML. 5) It has been demonstrated that the total Zinc score and the Zinc score gram serve a useful role as a supplementary diagnosis in identifying types (AML and ALL) of leukemias as well as in deciding the efficacy of the treatment. 6) In estimating similarly Zinc score of the peripheral blood granulocytes in Rauscher leukemic mice with lapse of time after inoculation of Rauscher virus, the total Zinc score decreased at time corresponding to the time when Rauscher virus began to proliferate in megakaryocytes in the spleen and as leukemia advanced, this value rapidly decreased. 7) These results seem to suggest that the decreasing mechanism of Zinc score in leukocytes intimately related to the onset as well as to the developmt of leukemia.