Fucoidan (Fucdn) and vitamin C (VC) were saturatedly dissolved in water and lyophilized and thoroughly ethanol-rinsed until no detection for supernatant vitamin C to form the Fucdn-VC (1:0.23 wt/wt) inclusion body (Fucdn-VC-IB). Fucdn-VC-IB increased not only VC-stabilizing at 37 degrees C, but also hydroxyl-radical scavenging as shown by ESR/spin-trap method, more markedly than a mere mixture of Fucdn:VC (1:0.23 wt/wt). Invasion of human fibrosarcoma cells HT-1080 through the basement membrane was repressed by Fucdn-VC-IB of non-cytotoxic concentrations without significant inhibition to human skin dermal fibroblasts DUMS-16 cells. Fucdn-VC-IB suppressed the invasiveness-related gelatinases MMP-2/9, and diminished reactive oxygen species inside the cytoplasm around the nucleus, in HT-1080 cells as shown by electrophoretic zymography and the redox indicator NBT assay, respectively. Thus Fucdn-VC-IB markedly exhibits antioxidant and MMP-suppressing activities and preferentially inhibited tumor invasion without cytotoxicity to normal cells, and is suggested as a potent tumor-invasion suppressor.