Lentinan is a purified β-glucan isolated from Lentinus edodes (shiitake mushroom). Past studies have shown that lentinan possesses strong inhibitory effects on tumors induced in mice via intravenous and intraperitoneal injection. The antitumor and immunomodulation effects of lentinan through oral administration were investigated in our study, and the results obtained proved its efficacy. The results showed that prefeeding of the mice for 7 days at an optimum dose of 3 mg/mouse was most effective against tumor induction, achieving a tumor inhibition rate (TIR) of 94.44%. Four key cytokines involved in the inhibition of tumor development-namely, interleukin-1α, interleukin-2, interferon-γ, and tumor necrosis factor-α-were induced significantly in the blood of the lentinan-fed mice. This also indicated the immunomodulating effect of lentinan. Further investigation showed that lentinan is capable of activating the lymphocytes of the host. Three types of immunodeficient mice-namely, nude mice (athymic mice), B-cell-deficient mice, and SCID (severe combined immunodeficient) mice-were used to demonstrate the protective capability of the activated lymphocytes. Normal AKR mice fed with lentinan for 7 days were sacrificed, and lymphocytes were isolated from their spleens. Adoptive transfer of these lymphocytes was done by inoculation via the immunodeficient mice tails. The sizes of the developed tumors (induced using the human colon carcinoma cells) were scored after 1 month. Large tumors were observed to be present in those immunodeficient mice that received lymphocytes from buffer solution-fed mice, whereas very small tumors or no tumors were observed in mice that received lymphocytes from lentinan-fed mice. The passively transferred lentinan-primed lymphocytes were able to restrict tumor development in these immunodeficient mice.