Phellinus linteus is a fungus distributed throughout Japan, Korea and China. Boiled water-soluble extracts from P. linteus (PLW) have shown anti-tumor and immunomodulatory properties in experiments done by intraperitoneal treatment, or in in vitro cell cultures. This is the first investigation on how oral administration of PLW influences immune responses. Here, we established immunodeficient mice by mitomycin C (MMC) and then researched how PLW influenced plaque-forming cell (PFC) production and populations of cytokine [interferon- (IFNγ-) and interleukin-4 (IL-4)]-producing T lymphocytes. PLW samples were administered orally for 19 days (1, 2 or 4 g/kg/day). PFC assay was followed using Jerne's method. IFN- and IL-4-producing T lymphocyte populations were measured by flow-activated cell sorter (FACS). These assays were conducted the day after the last oral administration. MMC groups were given MMC (1 mg/kg/day) intraperitoneally for 6 days with PLW administration. The number of PFC per 106 spleen cells increased significantly in the PLW (2 g/kg/day) group when compared with the MMC-control (P < 0.05) while populations of IFNγ- and IL-4-producing T lymphocytes decreased by MMC treatment. However, the PLW group tended to increase more than the MMC-control. Our results indicated that PLW augments the immune response of the spleen in MMC-induced immunodeficient mice.