Experimental infection with 0.01 LD50 P. aeruginosa 74 decreases the phagocytic activity of mouse neutrophils and peritoneal macrophages in relation to latex particles. In the course of infection suppression of phagocytic activity of peritoneal macrophages was delayed for 2 days as compared with that of neutrophils. During the dying out of infection, phagocytic activity of neutrophils and macrophages returned to the control values. Possible mechanisms of these events were discussed.