Coicis Semen is one of the oriental medicine that has been used for the treatment of the diseases such as pulmonary abscess, periappendicular abscess and wart since ancient times. However, the mechanism of the action of the drug is not well studied. This study was done to investigate the effects of Coicis Semen on the host defence mechanism. Effects of Coicis Semen on the immune responses were analysed by measuring the contact hypersensitivity, hemagglutinin, hemolysin and rosette formation, cytotoxicity, and reactive oxygen intermidiates production. As the results, water extract of Coicis Semen administration enhanced the antibodies (hemagglutinin and hemolysin) formation and the appearance of rosette forming cells of the spleen. Also Coicis Semen increased the allogeneic immune response in the mouse, showed cytotoxic activity against human leukemia cell line(K562) and decreased the contact hypersensitivity against dinitroflurobenzene. Also administration of Coicis Senlen slightly increased NK cell activity and enhanced the production of such reactive oxygen intermediates as superoxide and hydrogen peroxide from the macrophages in vivo and in vitro. The above results demonstrate that Coicis Semen has enhancing effects on cellular and humoral immune responses against disease.