Cellular reactions were studied through an investigation of cellular aspects of exudate formed with the application of Unguentum Vesicans to patients with chronic ieukemia. Following are the results. 1. As compared with normal controls, some delay was noted in the onset of blister formation and numbers of cells were less, but the decrease was not so remarkable as in acute leukemia. 2. Usually, the cells consist mostly of polymorphonuclear neutrophils, and in addition a few lymphocytes, monocytes, staff neutrophils, eosinophils, and basophils were noted. In chronic myelogenous leukemia, basophils were noted in higher percentage. 3. The average numbers of nucleus segmentation of the neutrophils were about the same as in normal controls. 4. In chronic myelogenous leukemia, wandering velocity of the neutrophils was decreased as compared with normal controls, but the decrease was not so remarkable as in acute leukemia. In chronic lymphocytic leukemia, the velocity was about the same as in normal controls. 5. In chronic myelogenous leukemia, cell numbers and wandering velocity of the neutrophils were remarkably increased during remissions as compared with the pre-treatment period.