Shosaiko-to (Xiao-chai-hu-tang, SHO), a Kampo medicine, was prepared by decocting a prescription of 7 kinds of crude drugs, namely Bupleuri Radix, Pinelliae Tuber, Scutellariae Radix, Zizyphi Fructus, Ginseng Radix, Glycyrrhizae Radix and Zingiberis Rhizoma. Previously, we reported that the effect of the orally administered SHO in augmenting natural killer (NK) activity in the peripheral blood was attributed to the acidic polysaccharide fraction.To characterize the active components in the crude materials in SHO, the effects of extracts and various fractions were investigated by oral administration. The extract of Zizyphi Fructus, Zingiberis Rhizoma, Scutellariae Radix, Glycyrrhizae Radix and Pinelliae Tuber augmented NK activity by oral administration. The high weight molecular fraction of Zizyphi Fructus was the most effective in augmenting NK activity. Thus, we obtained an active polysaccharide fraction from the high weight molecular fraction of Zizyphi Fructus. This polysaccharide fraction with a high molecular weight of approximately 43000 contained 54.7% carbohydrate, 61.8% uronic acid and 20.9% protein. The sugar moiety was composed of rhamnose, arabinose, xylose, fucose, mannose, galactose, glucose and galacturonic acid in molar ratios of 28 : 59 : 11 : 9 : 7 : 32 : 20 : 100.