Sulfated polysaccharides are known to be immune-stimulators in mammals and can be used as food additives to enhance immunity. In this study, the immune-stimulating activity of water-soluble anionic macromolecules F2 fractionation isolated from Codium fragile using ion-exchange chromatography was tested in olive flounder, Paralichythys olivaceus, in vitro and in vivo. The gene expression of interleukin (IL)-1β was adopted to check the immune-affection. As a result, in vitro study revealed that the expression of IL-1β was significantly upregulated in head kidney cells by 1 and 5 μg/ml of polysaccharides 4 h and by 5 μg/ml of polysaccharides at 24 h. In vivo, IL-1β gene expression in head kidney was significantly upregulated by 20 and 100 μg of the polysaccharides at day 1 post-i.p. injection, while downregulated at day 3 but not significant. Meanwhile, in peritoneal cells, it was upregulated by 20 μg of the polysaccharides at day 1 but the upregulation was sustained until day 3 though it was not significant. These results indicate that the sulfated polysaccharides from C. fragile are an immune-stimulator and might be potential feed additives for olive flounder.