A nonlectin glycoprotein (PCP-3A) newly isolated from the fruit body of edible golden oyster mushroom Pleurotus citrinopileatus has been shown to be growth inhibitory against human myeloid leukemic U937 cells in a previous report. There is a well-recognized relation between antitumor activity and immunomodulation. The immunomodulatory activity of PCP-3A was therefore assessed in the present study. Human mononuclear cells (MNC) and the CD4(+) T lymphocytes isolated from them were stimulated separately with PCP-3A for various durations and then filtered to obtain the conditioned media (CM). The conditioned medium from MNC (MNC-CM) was proved effective in inhibiting the growth of U937 cells. Increased secretion of cytokines TNF-α, IL-2, and IFN-γ from the stimulated MNC and CD4(+) T cells was found in CM. The antibody neutralization test of MNC-CM revealed that the growth inhibition on leukemic U937 cells was related to the elevation in cytokine concentration. We propose that PCP-3A stimulated human MNC to secrete cytokines TNF-α, IL-2, and IFN-γ, which subsequently inhibit the growth of U937 cells, and that PCP-3A may be a possible material for developing into an antileukemia ingredient in health food.