Effects of Lactobacillus casei YIT 9018 (LC 9018), which has antitumor activities against allogeneic and syngeneic murine tumors, on macrophage functions were examined. By intraperitoneal (i.p.) injection of LC 9018, acid phosphatase activity and phagocytic activity of peritoneal macrophages were enhanced significantly compared with those of normal peritoneal macrophages. The phagocytic activities showed peaks 2-3 days after the LC 9018-injection. LC 9018 accelerated the phagocytic function of the reticuloendotherial system in ICR mice tested by the carbon clearance test. The cytostatic activity of peritoneal exudate cells (PEC) induced by i.p. injection of LC 9018 into C57BL/6 mice against EL4 cells was also enhanced. On the other hand, PEC induced by L. fermentum YIT 0159, which has no antitumor activity, did not have cytostatic activity. These observations showed that LC 9018 was able to activate macrophages in mice.