1. The phosphohexose isomerase activity of the serum of patients with chronic myelocytic leukemia closely parallels the rise and fall in total granulocyte count.
2. Its separated leukocytes the isomerase activity of the granulocytes was found to be approximately three times that of the lymphocytes.
3. The serum isomerase activity was estimated in dogs with induced leukocytosis and leukopenia. The enzyme activity bore a closer relationship to leukocyte destruction than to leukocyte production.
4. These findings suggest that the elevated serum phosphohexose isomerase activity found in patients with chronic myelocytic leukemia probably originates from disintegrating granulocytes.