The adjuvant effects of Korean mistletoe lectin-C (KML-C) were investigated following the oral administration of KML-C with ovalbumin (OVA) as an antigen. Mice were orally immunized with OVA alone or admixed with various doses of KML-C or cholera toxin (CT), and the titer of OVA-specific antibody in the serum and mucosal secretions were determined. OVA+KML-C-treated mice showed high titers of IgA specific to CT in mucosal secretions. The antibody titers in the serum of OVA+KML-C-treated mice were comparable to those in the serum of OVA+CT-treated mice. When mice were immunized with OVA+KML-C or with CT alone and subsequently injected with OVA on the footpads after the primary immunization, they showed a more significant increase in delayed-type hypersensitivity reactions than when they were administered CT alone. These results suggest that KML-C is a potent immunoadjuvant that enhances both humoral and cellular immunity by the mucosal immune system.