Mushrooms have been shown to enhance immune response, which contributes to their antitumor property. White button mushrooms (Agaricus bisporus) (WBM) constitute 90% of the total mushrooms consumed in the United States; however, the health benefit of this strain in general is not well studied. Furthermore, little is known about WBM's immunologic effects. Dendritic cells (DC) are the most potent antigen presenting cells and play a pivotal role in immune response by linking innate and adaptive immune responses. In this study, we investigated the effect of in vitro supplementation with WBM on maturation of bone marrow-derived DC (BMDC) of C57BL mice. BMDC were differentiated in the presence of whole mushroom concentrate at 50, 100, or 200 mg/L. Results showed that mushroom supplementation dose dependently increased the expression of maturation markers CD40, CD80, CD86, and major histocompatibility complex-II. Consistent with the changes in the phenotypic markers, functional assay for DC maturation showed that mushroom supplementation decreased DC endocytosis and increased intracellular interleukin (IL)-12 levels. Furthermore, using a syngeneic T cell activation model, we found that WBM-supplemented DC from BALB/c mice presented ovalbumin antigen to T cells from DO11.10 mice more efficiently as demonstrated by increased T cell proliferation and IL-2 production. In conclusion, WBM promote DC maturation and enhance their antigen-presenting function. This effect may have potential in enhancing both innate and T cell-mediated immunity leading to a more efficient surveillance and defense mechanism against microbial invasion and tumor development.