A new water-soluble polysaccharide, APS-1II, with a molecular weight of 42.1 kDa was isolated from the roots of Angelica sinensis (Oliv.) Diels. APS-1II consists of arabinose (Ara), glucose (Glc) and fucose (Fuc) with a molar ratio of 2.48:1.05:1.00. The backbone of APS-1II is composed of 1,3-α-l-Araf and 1,6-α-d-Glcp with the branches containing 1,5-α-l-Araf, 1,4-β-d-Glcp, T-β-d-Glcp, 1,3-α-l-Fucp and T-α-l-Fucp. APS-1II inhibited the proliferation of human leukemia K562 and mouse L1210 cells in vitro and markedly prolonged the life span of L1210-bearing mice in vivo. APS-1II also increased the numbers of leukocytes and lymphocytes in peripheral blood, and significantly decreased plasma tumor necrosis factor, interleukin-2 and interferon-γ levels in L1210-bearing mice. Moreover, APS-1II administration concentration-dependently promoted the proliferation of the splenocytes, enhanced phagocytic activity of peritoneal macrophages and cytotoxicity of natural killer cells. These results suggest that APS-1II could effectively inhibit leukemia and induce a protective immune response, and it may be used as a suitable candidate reagent for leukemia therapy.