Expression of Siglec-9 was up-regulated on the development of monocytes into immature DCs and was decreased in mature DCs. Binding of various mucins and artificial glycopolymers carrying poly (NeuAc α2,6 LacNAc) or poly (NeuAc α2,3 LacNAc) to Siglec-9 was demonstrated by means of a plate assay. These mucins also bound to the surface of immature DCs. When immature DCs were treated with LPS in the presence of these mucins or artificial glycopolymers, the production of IL-12 was significantly reduced, but that of IL-10 was not. Furthermore, IL-12 production was decreased to a similar level on treatment with anti-Siglec-9 mAb. Mucins prepared from serum of cancer patients actually could bind to Siglec-9. These results suggest that Siglec-9 expressed on DCs is involved in immunoregulation through ligation with mucins in an epithelial cancer patient.