Abstract: The objective of this study was to separate polysaccharides from American ginseng by ultrafiltration and evaluate their immune enhancing activity. Hot water extract of the root was separated by membrane ultrafiltration into four different molecular weight fractions as follows: AGP1 ( > 100ku) , AGP2 ( 30¹00ku) , AGP3 ( 10³0ku) and AGP4 ( < 10ku) . Total yield of American ginseng polysaccharides was 5.17%, with the percentage of 46.61%, 3.30%, 12.11%, and 37.98% for AGP1^AGP4, respectively.Total sugar content of them was ranged from 42.87% to56.55%. Monosaccharide analysis showed that AGP1 ~ AGP4 were composed of six to eight monosaccharides, which all contained arabinose, rhamnose, mannose, glucose, and glucose acid. AGP1 and AGP2 showed remarkable immune enhancing activity by stimulating macrophages to produce NO, TNF-α, IL-6 and IL-10.