We investigated the antimetastatic effects of polysaccharides (RWP) isolated from radish leaves by hot water extraction and ethanol precipitation. From the sugar composition analysis, RWP was estimated to be a pectic polysaccharide. Peritoneal macrophages stimulated with RWP significantly augmented the production of various cytokines. In an assay for natural killer (NK) cell activity, both intravenous (i.v.) and oral administration of RWP significantly enhanced NK cells cytotoxicity against YAC-1 lymphoma. In an experiment on lung metastasis of Colon26-M3.1 carcinoma, prophylactic i.v. and oral administration of RWP inhibited metastasis by 77.2% and 55.0%, respectively, at 1000 μg/mouse. Furthermore, pretreatment with anti-asialo GM1 for NK cells depletion partly suppressed the inhibitory effect of RWP on lung metastasis. Thus, our study results suggest that RWP has superior antimetastatic activity, and this effect is related to the activation of various immune factors.