Human mast cells and basophil granulocytes can be easily recognized in normal tissues by light microscopy. In one mast cell and one basophilic leukemic case considered in this study, mast cells and basophils were morphologically quite similar and could not therefore be clearly defined merely by their morphological features. Both types of cells showed round nuclei and deep purple granules. The diagnosis of mast cell leukemia or basophilic leukemia was made on the basis of different cytochemical patterns. In the case of mast cell leukemia, peroxidase and PAS stains were negative, while chloroesterase was strongly positive; in the case of basophilic leukemia, peroxidase and PAS stains were positive, while chloroesterase reaction showed a peculiar pattern. Toluidine blue metachromasia and astra blue positivity were present in the cells of both cases.