To examine the new practical utilization of mucilages in Opuntia humifusa, the Korean pear cactus, the polysaccharides were isolated from O. humifusa, and immuno-stimulating activities were assayed. The main polysaccharide, CNC-E, was prepared by a commercial enzyme treatment, water extraction, and ethanol precipitation. The molecular mass of CNC-E was estimated to be about 700 kDa, and it consisted mainly of arabinose, galactose and xylose in addition to two minor sugars such as rhamnose and fucose. On the other hand, CNC-E showed considerably high splenocyte proliferation activity in a dose-dependent manner. Peritoneal macrophages stimulated with CNC-E produced cytokines such as IL-6, IL-12, IL-10, and TNF-${alpha}$. The intravenous administration of CNC-E significantly augmented the cytotoxicity of natural killer (NK) cells against Yac-1 tumor cells. Especially, NK cells obtained from the mice treated with $100{mu}g$ of CNC-E showed threefold higher cytolytic activity than those of untreated mice. CNC-E also showed potent anti-complementary activity in a dose-dependent fashion. Identification of C3 activation products by the crossed immunoelectrophoresis using anti-human C3 and the anti-complementary activity of CNC-E in a $Ca^{2+}$-free condition suggested complement activations by CNC-E that occur via both alternative and classical pathways. These results indicate that Korean pear cactus contains selected polysaccharides that provide immuno-stimulating activities beneficial to human health.