Cellular reactions were studied through an investigation of cellular aspects of exudate formed with the application of Unguentum Vesicans to patients with acute leukemia and monocytic leukemia. Followings 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. The decrease was especially remarkable up to 6 hours following the onset of blister formation and in some cases, acellular exudate was noted. 2. The cells consist mostly of polymorphonuclear neutrophils, and in addition a few lymphocytes, monocytes, and staff neutrophils were noted. Apperance of immature cells was noted in exceptional cases. 3. The average numbers of nucleus segmentation of the neutrophils were less as compared with normal controls. 4. Wandering velocity of the neutrophils was remarkably decreased as compared with normal controls. 5. In acute lymphocytic leukemia and monocytic leukemia, numbers of cells were increased during remissions.