We examined the relationship between natural killer (NK) cell-mediated cytotoxicity, the produced active-oxygen and cytotoxic factor (CF) release in co-culturing canine NK cells with tumor cells (CL-1 target cells). In co-culturing, the adding of n-propyl gallate (active-oxide scavenger) removed the produced active-oxygen, which inhibited NK cell-mediated cytotoxicity and the CF release. Moreover, adding of this agent inhibited the tyrosine phosphorylation of NK intracellular protein which observed in co-culturing. Therefore, the active-oxygen produced from canine NK cells are thought to relate the signal transduction in NK-mediated cytotoxicity.