King oyster mushroom (Pleurotus eryngii), an improved species of oyster mushroom, is a popular ingredient in Asian cuisine. Spleen cells were treated with various concentrations (0, 5, 10, 50, 100, 250, 500, and 1,000 μg/mL) of king oyster water extracts (KOWE); then, the proliferation of the cells was measured 24, 48, and 72 h after each treatment. Also, type 1 T helper cytokine productions (TNF-α, IFN-γ, and IL-2) were measured in activated macrophage by KOWE in seven concentrations. Under the condition of its 50, 100, 250, and 1,000 μg/mL for 48 h, the proliferation of cells was increased. However, there was no significant fluctuation in the spleen cells proliferation for 24 and 72 h-long KOWE exposure. To determine cytokine (TNF-α, IFN-γ, IL-2) productions of type 1 T helper cells, macrophage was stimulated by KOWE for 48 h. Treatment of KOWE gave a rise to the levels of TNF-α and IFN-γ, but not in that of IL-2 productions. These results suggest that king oyster mushroom water extracts may be beneficial for enhancing immune functions in its high concentration.