Edible mushrooms contain an abundance of immune-enhancing nutritients. Some of these compounds, referred to as biological response modifiers (BRMs), have been used in biological therapies for cancer treatment. We obtained a low-molecular-weight protein fraction (MLP-Fraction) from the fruiting body of the maitake mushroom Grifola frondosa by multiple sequential steps, including ethanol precipitation, DEAE-exchange chromatography, and gel filtration. The effect of the MLP-Fraction on the immune system was determined using normal mice. This resulted in a simultaneous increase in splenocyte proliferation and production of cytokines such as interleukin (IL)-1alpha, tumor necrosis factor-alpha, IL-10, IL-12, and interferon (IFN)-gamma. The expression levels of IFN-gamma and IL-12 in antigen-presenting cells (APCs) and the activation of natural killer (NK) cells, macrophages, and dendritic cells were observed. These results suggest a mechanism in which NK cells are activated through cytokines produced by APCs. We also confirmed the possibility that the MLP-Fraction acts as a BRM using colon-26 carcinoma-bearing mice. This fraction enhanced the production of IL-12 and IFN-gamma by splenocytes in tumor-bearing mice and clearly showed an inhibitory effect on tumor cell growth.