In order to study the defense reaction of the body to a local infection, an investigation was made for cellular aspects of exudate formed with the application of Unguentum Vesicans. Followings are the results. 1. The onset of blister formation averaged 8,7 hours following the application of Unguentum Vesicans. 2. Apperance of cells was noted with an increase of the exudate following blister formation. Numbers of cells in the exudate in 12 cases 1,3,6 and 12 hours following the onset of blister formation averaged 1,037, 1,481, 1,245, and 2,083 respectively. 3. The cells conist mostly of polymorphonuclear neutrophils, and in addition a few lymphocytes, monocytes, eosinophils, and staff neutrophils were noted. 4. The average numbers of nucleus segmentation of the neutrophils in 12 cases 1,3,6, and 12 hours following the onset of blister formation were 2.85, 3-08, 3.14, and 3.32 respectively, showing a constant increase with passage of time. 5. Wandering velocity of the neutrophils in 12 cases 1,3,6, and 12 hours following the onset of blister formation averaged 10.04μ/min, 12.57μ/min, 10.76μ/min, and 11.06μ/min respectively, showing a relatively active movement constantly. 6. An investigation of the exudate formed with the application of Unguentum Vesicans has a merit in that calculation of cell numbers and measurement of the wandering velocity can be performed with ease.