1. Serum lactic dehydrogenase (SLD) was measured in 66 patients with leukemia. Elevated levels were observed in 23 (77 per cent) of 30 adults with acute leukemia; in 7 (70 per cent) of 10 children with acute leukemia; in 15 (94 per cent) of 16 patients with chronic myelocytic leukemia, and in 6 (60 per cent) of 10 subjects with chronic lymphatic leukemia.
2. The degree of SLD elevation appeared to be significantly greater in the chronic myelocytic than in the chronic lymphatic group.
3. Serial determinations of SLD were performed for periods of from 1 to 82 weeks in 55 patients, of whom 43 had elevated levels at one time or another. Fair to good correlation between SLD and conventional parameters of leukemic activity was noted in 7 (33 per cent) of adult acute leukemias; in 2 (40 per cent) of child leukemias; in 8 (67 per cent) of chronic myelocytic leukemias; and in 1 (20 per cent) of the chronic lymphatic group.
4. Possible sources of enzymes are discussed. At the present time the mechanism of SLD elevation in leukemia remains an unsolved problem.
5. Serum transaminase (SGOT) levels were studied in 61 subjects. Levels greater than 100 units were seen in only 8 patients, of whom 7 had either viral or toxic hepatitis and 1 had terminal septicemia. Other types of liver impairment, including leukemic infiltration, were associated with normal or slightly elevated SGOT.