Two polysaccharides (WEA and WEB) were isolated from Enteromorpha intestinalis by hot water extraction, anion-exchange, and gel-permeation chromatography. The average molecular weights (Mw) of the two fractions were 72.03 kDa (WEA) and 60.12 kDa (WEB). WEA was composed of Rha, Xyl, Man, and Glc in a molar ratio of 1.39:1.00:0.13:3.23. WEB consisted of Rha, Xyl, Gal, and GlcA (glucuronic acid) in a molar ratio of 7.32:1.00:0.51:1.28. Both polysaccharides could inhibit tumor growth in S180 tumor-bearing mice, and increased the relative spleen and thymus weight. They also increased the expression of tumor necrosis factor-alpha (TNF-${alpha}$) in serum. WEA and WEB induced lymphocyte proliferation, increased the production of TNF-${alpha}$ in macrophages, and stimulated macrophages to produce nitric oxide dose-dependently through the up-regulation of inducible NO synthase activity. However, no direct cytotoxicity against Sarcoma 180 was investigated in vitro. These results indicate that the antitumor effects of these polysaccharides are associated with immunostimulation.