Mushroom extracts have long been used in Asia as immunopotentiating agents. Previous studies have shown that ingestion of mushroom extracts can cause tumors to shrink or stop progressing in animal models of cancer and in cancer patients. However, how these agents induce anti-tumor effects is unknown. The proposed study will use the neutransgenic (neu-tg) mouse, a model of breast cancer, to study the effects of protein-bound polysaccharide Krestin (PSK), an extract from the woody mushroom Trametes versicolor, on anti-tumor immune responses. The goal of this study is to determine in the neu-tg model whether PSK induces an anti-tumor immune response, thus reversing immune suppressive effects of the tumor microenvironment.