A fucoidan, isolated from Korean Undaria pinnatifida spoprophyll (UP-F), was investigated for its immunomodulating activity on murine macrophages and splenocytes, and its activity was compared with that of fucoidan from Fucus vesiculosus (FV-F). Treatment of UP-F resulted in inhibition of the growth of murine macrophage RAW 264.7 cells, but its cytotoxicity was not observed in normal murine splenocytes. FV-F was shown to be highly cytotoxic to both immune cells, and its cytotoxic activity was higher than that of UP-F. Treatment of UP-F induced TNF-α in a dose-dependent manner from two types of macrophages, RAW 264.7 cells and murine peritoneal macrophages. The TNF-α-inducing activity of UP-F was higher than that of FV-F. UP-F also actively induced chemokines (RANTES and MIP-1α) from RAW 264.7 cells. Furthermore, treatment of UP-F gave rise to activation of murine splenocytes to produce cytokine (IL-6) and chemokines (RANTES and MIP-1α), showing significantly higher activity than that of FV-F. These results indicate that UP-F is less cytotoxic to immune cells than FV-F, and possesses immunomodulating activity to produce cytokines and chemokines from macrophages and splenocytes.