It has been known for many years that most fungi which have known effects against cancers of the stomach, esophagus, lungs, etc., belong to the Polyporacea. However, the components responsible for such action have not been clearly defined. In Japan, in 1968, it was reported that a hot water extract from some edible mushrooms belonging to the Polyporacea showed a marked host-mediated antitumor activity. Since then, numerous researchers have isolated active polysacchareide and have identified them to be (l→3)-β-D-gluco-pyranans with a (1→6)-β-D-glucosyl branch containing protein. Several antitumor polysaccharides, some hetero-β-glucans and its protein complexes such as xyloglucans, acidic β-glucan containing uronic acid, were isolated from the extracts by large amounts of dilute alkali but are insoluble water in Japanese mushrooms (8 species) and Chinese mushrooms (5 species). Several trials have been made to enhance activity by chemical modification such as poly-alcohols formed by a mild Smith degradation and the products formed by BH_4-reduction after IO_4-oxidation. Mushroom polysaccharides are considered to be biological response modifiers (BRM) or immunopotentiators because of their action mechanism. In Japan three different polysaccharide antitumor agents have been developed from the fruiting body, mycelium, and cultured medium, from three mushroom species.