Leukocyte alkaline phosphatase activity was determined in 38 patients with chronic granulocytic leukemia. One quarter of the patients who achieved remission had normal leukocyte alkaline phosphatase activity. Three patients in the blastic phase of the disease and three patients who were inadvertently rendered aplastic by chemotherapy had marked increases in enzyme level. These observations suggest that the potential for the production of this enzyme in some patients with chronic granulocytic leukemia is not permanently lost. The source of this increase in leukocyte alkaline phosphatase activity is unknown.