An attempt has been made at the cytological study on normal ascites and thoracic fluid obtained by puncture from man and various animals, by counting number of cells in respective fluid, by preparing smeared-stained specimens, and by performing peroxidase reaction. The results are as follows: 1. As for the cell count in ascites, in mouse it was 74,100, in rat 67,221, in rabbit 1,795, in dog 3,040, and in man 1,161/㎣ , respectively; while the count in thoracic fluid, in mouse it was 68,337, in rat 75,560, in rabbit 1,555, it dog 3,177, and in man 1,163/㎣ , respectively. 2. As for the cell composition in every case phagocytes occupy the major portion as high as 80.6-92.5 per cent of the total cells, and the remainder consists of a few of lymp...