Synthesis of nitric oxide (NO) is one of the important effector functions of innate immune cells. Although several reports have indicated mistletoe lectins induce immune cells to produce cytokines, studies regarding the activities of the lectins in the production of NO have been very limited. Here, we report on the induction of NO synthesis in a murine macrophage cell line, RAW264.7, by Korean mistletoe lectin (KML-IIU). When the macrophage cells were treated with KML-IIU in the presence of a suboptimal concentration of IFN-gamma, NO production was induced in a concentration-dependent manner. Significantly higher levels of NO were induced by subchains of the KML-IIU (A and B), which have lower toxicities, as compared to the hololectin. Furthermore, expression of the inducible nitric oxide synthase (iNOS) gene was elevated in accordance with the level of NO production. When the synthase was inhibited by iNOS inhibitors (L-NIL and L-NAME), NO production was specifically reduced in a concentration-dependent manner. Our studies demonstrate that the KML-IIU and its subchains induce NO production in murine macrophage cells via activation of the iNOS gene expression, suggesting that the KML-IIU subchains may be used as an immunomodulator to enhance the effector functions of innate immune cells.