1. Serum lactic dehydrogenase (S-LDH) activity and isozyme were measured in 29 patients with leukemia. The elevated S-LDH levels were demonstrated in all cases of acute leukemias and chronic myelocytic leukemias and in a half of cases of chronic lymphocytic leukemias. The S-LDH isozyme can be identified five fractions by means of agar gel electrophoresis. The changes of pattern of S-LDH isozyme were observed in all types of leukemia. It was the increased third (LD(3)), second (LD(2)) and first (LD(1)) fractions of LDH isozyme and the decreased fifth fraction (LD(5)). The most significant fraction in leukemia was LD(3), and the increased LD(3) and decreased LD(5) were observed in 25 of 29 cases (86 % ). 2. The degree of S-LDH elevation appeared to be the greatest in the acute lymphatic and the smallest in the chronic lymphatic leukemia. The degree of increase of LD(3) was independent on the type of leukemia. 3. As to the correlation of S-LDH with clinical findings, the level of S-LDH and LD(3) well correlated with the percentage of circulating immature leukocytes, but not with the absolute number of leukocyte. 4. The serial S-LDH levels and LD(3) closely paralleled with the clinical courses in 10 cases of 29 patients with leukemia and relatively well paralleled in 13 cases. 5. In regard to possible mechanisms of S-LDH elevation in leukemia, acceleration of biosynthesis of LDH in the leukemic cells and liberation of the enzyme from the cells might be suggested.