Agaricus bisporus white button mushroom (WBM) is widely consumed in most countries for its culinary properties. Recently, its dietary intake has been shown to protect against breast cancer. Mushroom polysaccharides are known for their immunomodulating and antitumor properties; however, little is known regarding the properties of A. bisporus polysaccharides. Using size-exclusion chromatography to fractionate the crude extract of A. bisporus, two polysaccharide fractions (designated as ABP-1 and ABP-2) were obtained. The estimated molecular masses of ABP-1 and ABP-2 were 2,000 kDa and 40-70 kDa, respectively, and their sugar compositions consisted mainly of glucose, mannose, xylose, and fructose. Analysis of the effects of the polysaccharides on murine macrophages demonstrated that both fractions stimulated the production of nitric oxide, interleukin-6, and tumor necrosis factor-α. Modulation of macrophage function by A. bisporus polysaccharides was mediated in part through activation of nuclear factor-κB with the production p50/105 heterodimers. Both ABP-1 and ABP-2 had the ability to inhibit the growth of human breast cancer MCF-7 cells but had little effect on the growth of human colon, prostate, gastric cancer, and murine Sarcoma 180 cells as assessed by a tetrazolium dye [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide]-based assay. However, when murine Sarcoma 180 cells exposed to ABP-1 or ABP-2 were implanted subcutaneously into mice, a reduction in tumor growth was observed compared with that observed in control mice. Taken together, our data provide a molecular basis to explain in part the reported beneficial therapeutic effects of A. bisporus WBM intake and suggest that macrophages likely contribute to the antitumor effects of Agaricus polysaccharides.