By addition of hydrochloric acid or causatic soda at various concentrations to bone-marrow tissue culture (cover-slip method) of rabbits, the author studied the influences of medium pH on the motility of pseudoeosinophils and obtained the following results: 1. The limit of pH of the medium which enables such observations of the movement of pseudoeosinophils is pH 7.11-8.05. 2. The more acidic or basic is the medium, the more marked is the decrease in the wandering velocity of pseudoeosinophils. 3. In the medium both acidic and basic, the boundary of the cell is distinct; the shape is irregular with different sizes of granules; pseudopodia are small; and the wandering cycle is indistinct. 4. In the medium which is either highly acidic or basic, the number of cells of Types A and B are less, but those of Types C, E, and I are increased. 5. By changing pH of the medium either higher or lower those with pseudopodia shaped like a tongue are lessened and the cells with saw-like pseudopodia are increased. Moreover, this tendency is stronger as the pH approaches that of the control medium; and what is more it is markedly so in an earlys tage of culture. 6. The tail-shape pattern of Type Ⅰ can be found less in the medium of pH 7.36 or pH 7.80, but this type increases in both more acidic and more basic media. Type Ⅱ shows a tendency just the reverse of Type Ⅰ. Type Ⅲ is rare in any medium, but it tends to increase slightly with the lapse of culture time. 7. As for the ratio of the long axis and short axis of a pseudoeosinophils, the smaller ratio increases while the greater ratio decreases, in a more acidic or a more basic medium.