A study to evaluate the mechanisms of tumoricidal activity resulting from orally administered extract of Agaricus blazei Murill (A. blazei) was performed in mice bearing syngeneic and xenogeneic tumors. Tumor regression was comparably seen in both syngeneic and xenogeneic tumor-bearing mice when administered oral extract preparations. In addition, in a murine syngeneic tumor model, oral administration of water-soluble extracts of A. blazei resulted in significant production of cytokines such as IFN-γ, and TNF-α in peritoneal exudate cells, in parallel with the marked regression of tumor development. The water-soluble extracts also induced pronounced antioxidant activity in in vitro and in vivo assays using two different methods. These results indicate the A. blazei extract may enhance not only the immnunomodulatory effects that promote activity of peritoneal exudate cells for tumor regression but also potentially result in the direct destruction of tumor cells through its antioxidant activity.