Corynebacterium liquefaciens can induce a marked resistance to tumor cell (AH-7974) challenge in Donryu rats. This effect was also induced in T-cell-deficient athymic nude mice. The macrophages harvested from the peritoneal cavity of a rat injected with C. liquefaciens showed a marked inhibition of tumor growth in vitro. Therefore, the effector cells were assumed to be peritoneal macrophages. The time needed for activation of macrophages in vivo was within 3 to 5 days after injection of C. liquefaciens. Before or after this period, the peritoneal macrophages did not inhibit tumor cell proliferation. Furthermore, normal peritoneal macrophages were activated in vitro by C. liquefaciens and displayed remarkable antitumor activity. These results suggest that C. liquefaciens rendered the macrophages inhibitory to tumor cells even under T-cell depleted condition. On the other hand, in vitro antitumor activity of the activated macrophages disappeared completely after in vivo treatment of the peritoneal cavity of a rat with Trypan Blue, a lysosomal enzyme blocker. Resistance of the rat induced by C. liquefaciens to tumor cells was also abrogated by Trypan Blue treatment just 1 day before tumor cell inoculation. Trypan Blue blocked the acid-phosphatase (a marker enzyme of lysosomes) activity of the activated macrophages but it seemed not to affect other macrophage activities, at least in the following two points; normal macrophages treated with Trypan Blue had almost normal phogocytic function of India ink particles and, moreover, they could be rendered tumoricidal, same as normal macrophages, in vitro by C. liquefaciens.