Abstract Cytochalasin B (2 to 4 µg/ml) reversibly inhibits the chemotactic response of human and rabbit polymorphonuclear leukocytes to a bacterial chemotactic factor or to the complement derived factor, C5a. At lower concentrations (1 to 0.1 µg/ml or less) Cytochalasin B actually stimulates chemotaxis. Spontaneous movement of the leukocyte is reversibly inhibited at concentrations as low as 0.2 µg/ml. Enhancement of the spontaneous motility has not been observed at this or lower concentrations.