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Depolymerization of κ-carrageenan was performed using carrageenase isolated from the cell-free medium of a culture of marine Cytophaga sp. MCA-2. The low-molecular-weight carrageenans after ultrafiltration and lyophilization were sulfonated with formamide-chlorosulfonic acid. The anti-tumor activity of the products with different molecular weight was determined by using Sarcoma 180 tumor in mouse. A carrageenan oligosaccharide with a molecular weight of 1726, administered orally at a dose of 100 mg kg−1 mouse markedly inhibited tumor formation. However, the anti-tumor activity of high-sulfonated carrageenan was much less than that of the non-sulfonated or light-sulfonated preparation. The activities of the latter products on superoxide dismutase and catalase were enhanced considerably, which suggests that carrageenan oligosaccharide was effective in promoting the antioxidation ability and eliminating danger from free radicals. The preparations showed special effects on immunological regulation, especially the phagocytosis ratio and phagocytosis index of macrophage, which might be beneficial for the anti-tumor activity. Although no anti-tumor activity of this product was detected in vitro, suggesting that its activity differs between in vitro and in vivo, this 1726 molecular weight product provides a potent clinical use in tumor treatment. [ABSTRACT FROM AUTHOR]