Fucoidans, high-molecular-weight sulfated polysaccharides extracted from brown seaweeds, have various biological activities. Here we examined whether fucoidan could ncrease the immunomodulation and proliferation capacity of cells in vitro. When peritoneal macrophages were treated with various concentrations of fucoidan $(1~100{mu}g/mL)$ for 24 hours, NO production was significantly increased. In addition, exposure of macrophages to $10{mu}g/mL$ of fucoidan induced a phagocytic activity. Treatment of neuroblastoma cells (SK-N-SH) with fucoidan enhanced cell proliferation and NO production in a concentration-dependent manner. These results indicate that fucoidan has both immunomodulatory and cell proliferative properties, and may thus be a candidate for development as an immunomodulating agent.