1. The results of over 400 assays of fumarase activity in separated leukocytes and erythrocytes in a variety of hematologic and nonhematologic diseases are presented.
2. The highest mean leukocyte fumarase values occurred in acute monocytic leukemia and acute granulocytic leukemia. Blast cells have greater fumarase activity than more mature forms of the same cell type.
3. Fumarase appears to be distributed ubiquitously among the various leukocytes. Cell populations which contained predominantly neutrophils, lymphocytes, monocytes, blasts, eosinophils, and basophils, all exhibited appreciable enzyme activity.
4. Assays of erythrocyte fumarase activity did not reveal a pattern with the exception of elevated values in macrocytic anemias and in some, but not all, cases of hemolytic anemia.