To investigate mechanism of antineoplastic activities of the protein-polysaccharide fraction of Coriolus versicolor (Fr.) Quel., the mycelium of the fungus was grown in a liquid medium and was extracted with hot water. The extract was purified and used for examining its effects upon immune response in mice. The fraction was administered at a dose of 20mg/kg/day to ICR mice for five days. After ten days the mice were immunized by sheep red blood cells. The plaque-forming cells (PFC) in their spleen were increased in the treated mice. The number of PFC in case of the protein-removed polysaccharide was lower than that of the entire protein-­polysaccharide fraction of the fungus.