The antitumor and metastasis-inhibitory activities, mode of action, and clinical application of lentinan, a strictly purified beta-1,6:beta-1,3-glucan, are reviewed. Lentinan exerts a prominent antitumor effect and prevents chemical and viral oncogenesis. The antitumor action of lentinan is host-mediated. Compared to other well-known immunostimulants, such as bacille Calmette Guérin (BCG), Corynebacterium parvum, and lipopolysaccharide (LPS), lentinan appears to represent a unique class of immunopotentiator, a T cell-oriented adjuvant. Lentinan triggers the increased production of various kinds of bioactive serum factors associated with immunity and inflammation, such as IL-1, CSF, IL-3, vascular dilation inducer, and acute-phase protein inducer, by the direct impact of macrophages or indirectly via lentinan-stimulated T cells, which results in the induction of many immunobiological changes in the host. Augmented IL-1 production amplifies the maturation of immature effector cells to mature cells capable of responding to lymphokines such as IL-2 and T cell-replacing factors. Because of this mode of action, intact T cell compartments for antitumor activity of lentinan are required. Lentinan has little toxic side effects. Excellent results were obtained in a 4 year follow-up of the randomized control study of lentinan in phase III on patients with advanced and recurrent stomach and colorectal cancer.