To obtain an immunosuppressant capable of solving the problems of adverse reactions of conventional immunosuppressants such as a cytotoxicity, a reduction in infection resistance, a suppression of the production of bone marrow hemopoietic cells, or the like, and providing a new usage such as a graft versus host reaction inhibitor, or the like, by using a polysaccharide component obtained from fruit bodies of Agaricus blazei.

This immunosuppressant is a polysaccharide component obtained as a hot water soluble component from fruit bodies of Agricus blazei and used for suppressing immune reactions. The polysaccharide component is preferably obtained by treating the fruit bodies of Agricus blazei with a heated 80-85% ethanol for 18-22 hr, then repeating the same process at least 3 times, extracting the above extraction residue with a hot water for 18-22 hr, repeating the hot water extraction for at least 3 times and lyophilizing for concentrating the obtained hot water soluble component. In an immunosuppressive treatment, administrations by an oral or injection can be adopted. Usually, the polysaccharide component is administered in a dosage of 5-100 mg/kg body weight. In a functional food, it can be added by 0.001-0.1 wt.%.