Despite a great deal of effort made by many researchers throughout the world, chemotherapeutic agents that attack cancer cells directly do not seem to have the expected effects except on some leukaemias. Besides, these agents show strong toxicity to the host, and reduce the host defense against infections, especially destroying lymphocytes and bone marrow cells. To find a new cancer drug that can activate or restore host defense mechanisms, we examined fungi, which had traditionally been said to be effective against cancer in Japan and other Asian countries, such as Ganoderma applanatum (Pers.) Pat. and Coriolus versicolor (Fr.) Quel., and several kinds of Japanese edible mushrooms. Test substances were administered intraperitoneally and screened for their ability to inhibit the growth of sarcoma 180 cells subcutaneously transplanted into swiss or ICR mice. This method, reported by Nakahara et al. [1], has been proven to be simple and suitable for screening of host-mediated anticancer drugs. Table 1 shows various antitumoral polysaccharides including lentinan that were isolated from fungi, basidiomysetes and yeast. Lentinan, a (1→3)-β-D-glucan with (1→6)-β-D-glucopyranoside branches isolated from an edible mushroom, Lentinus edodes (Berk.) Sing., exhibits a marked antitumor effect against sarcoma 180 cells transplanted subcutaneously at a dose of 1 mg/kg/day for ten days (Fig. 1) [2–4]. Its chemical and physical characteristics are listed in Table 2.