The composition of monosaccharides of acidic polysaccharide isolated from ethanol-insoluble and water-soluble fractions of red ginseng roots was analysed and its immunological activities were investigated. Red ginseng acidic polysaccharide (RGAP) was composed of glucose (26.1 mole %), arabinose (1.6 mole %), glucuroninc acid (51.8 mol %) and galacturonic acid (5.1 mole %) as determined by gas liquid chromatography. Addition of RGAP increased production of nitric oxide (NO) and tumor necrosis factor (TNF)-$alpha$ in the rodent macrophage cultures. Peritoneal macrophages from RGAP-treated mice exhibited potent tumoricidal activities toward P815 and WEHI 164 tumor cells. It was also observed that concentrations of NO and TNF-$alpha$ were high in the culture medium of macrophages from the mice administered with RGAP. Moreover, treatment of RGAP in vivo stimulated tumoricidal activities of natural killer (NK) cells. Treatment with RGAP increased life span of sarcoma 180-bearing mice and decreased tumor weights of B16-tumor-bearing mice. These results suggest that activation of macrophages and NK cells serve to enhance in vivo anticancer activities of RGAP.