Mushrooms are reported to possess antitumor, antiviral, and antibacterial properties. These effects of mushrooms are suggested to be due to their ability to modulate immune cell functions. However, a majority of these studies evaluated the effect of administering extracts of exotic mushrooms through parental routes, whereas little is known about the immunological effect of a dietary intake of white button mushrooms, which represent 90% of mushrooms consumed in the U.S. In this study, we fed C57BL/6 mice a diet containing 0, 2, or 10% (wt/wt) white button mushroom powder for 10 wk and examined indices of innate and cell-mediated immunity. Mushroom supplementation enhanced natural killer (NK) cell activity, and IFNgamma and tumor necrosis factor-alpha (TNFalpha) production, but only tended to increase IL-2 (P = 0.09) and did not affect IL-10 production by splenocytes. There were significant correlations between NK activity and production of IFNgamma (r = 0.615, P < 0.001) and TNFalpha (r = 0.423, P = 0.032) in splenocytes. Mushroom supplementation did not affect macrophage production of IL-6, TNFalpha, prostaglandin E(2), nitric oxide, and H(2)O(2), nor did it alter the percentage of total T cells, helper T cells (CD4(+)), cytotoxic or suppressive T cells (CD8(+)), regulatory T cells (CD4(+)/CD25(+)), total B cells, macrophages, and NK cells in spleens. These results suggest that increased intake of white button mushrooms may promote innate immunity against tumors and viruses through the enhancement of a key component, NK activity. This effect might be mediated through increased IFNgamma and TNFalpha production.