In the bone-marrow tissue culture (cover-slip method) of rabbits, the author studied the effects of osmotic pressure of medium on the motility of pseudoeosinophils, and arrived at the following conclusions: 1. The osmotic pressure of the culture medium that enables such observations of the movement of cells has been found to be, when calculated in terms of the saline concentrations, 0.57% the minimum, and 1.48% the maximum. 2. The growth zone in the hypertonic medium shows a high celldensity with a clear-cut boundary, and the growth zone in the hypotonic medium reveals a markedly thin cell-density. 3. The wandering velocity in the hypotonic medium at various osmotic pressures does not alter so markedly, showing an acceleration in an early stage but a precipitous fall later. In the hypertonic medium, the more hypertonic the medium is, the more marked is the wandering velocity. 4. In the early stage of the hypertonic medium there appears a specific form of cells possessing sharp needle-shaped pseudopodia with hardly no difference in the long axis and short axis, and the more hypertonic the medium is the greater is the number of such peculiar cells. 5. There is a tendency of an increase in the number of Type D of the movement patterns in the hypotonic medium, though the number is small. 6. No marked changes can be observed in the tail-shape of cells due to changes in osmotic pressure of the medium. 7. The more hypertonic or the more hypotonic the osmotic pressure of the medium is, the smaller is the ratio of the long axis and the short axis of pseudoeosinophls.