Sulfated polysaccharides isolated from Ulva pertusa and fractionated using anion-exchange chromatography were investigated to determine their molecular characteristics and bioactivities. The crude and fractionated polysaccharides (F(1), F(2), and F(3)) were mainly composed of carbohydrates (59.9-65.9%), sulfates (11.6-15.3%), and uronic acid (7.30-16.4%) with small amounts of proteins (1.40-4.80%). Rhamnose (62.5-80.7%) was the major monosaccharide unit of these polysaccharides, with different levels of glucose (13.5-27.4%) and xylose (2.74-11.5%). The polysaccharides contained one or two major subfractions with weight-average molecular mass ranging from 51.1×10(3) to 1,690×10(3) g/mol. The relatively low in vitro anticancer activity of the polysaccharides (22.3-42.4%) suggested that they had little cytotoxicity against the cancer cell line used (AGS). On the other hand, the polysaccharides significantly stimulated Raw 264.7 cells, inducing considerable amounts of nitric oxide and various cytokines production, which suggested that they could be strong immunostimulators.