Agaricus blazei Murill has been reported to possess biological effects that include immunomodulatory and tumoricidal activities, but its potential effects have not been systematically analyzed in vivo. We evaluated the immunomodulatory effects of A. blazei in Balb/cByJ mice.160 male Balb/cByJ mice were divided into four groups and treated with various quantities of intragastric A. blazei extract or distilled water for 8 to 10 weeks. Nine parameters, relating to general immune function or adaptive immunity against immunogen chicken oval albumin, were determined.The results revealed that mice receiving A. blazei extract exhibited significantly greater serum immunoglobulin G levels, increased T-cell numbers in spleen, and elevated phagocytic capability compared with controls. Consumption of A. blazei was also associated with significant increases in oval albumin-specific serum immunoglobulin G level, delayed-type hypersensitivity, splenocyte proliferation rate, and tumor necrosis factor-alpha secretion by splenocytes.Consumption of A. blazei extract was associated with significant enhancement of seven out of nine immune functions tested. We conclude that A. blazei Murill possesses a wide range of immunomodulatory effects in vivo.