The immunomodulatory effect of GLIS (Lingzhi or Reishi medicinal mushroom Ganoderma lucidum immunomodulating substance) on macrophages has been investigated as part of ongoing research into the anticancer properties of this mushroom. Proliferation of bone marrow macrophages (BMMs) was enhanced by GLIS in a dose-dependent manner. Microscopic examination revealed that numerous GLIS-treated BMMs were enlarged and formed pseudopodia. Exposure of BMMs to GLIS resulted in significant increases in NO production, induction of cellular respiratory burst activity, and increased levels of IL-1β, IL-6, IL-12p35, IL-12p40, IL-18, and TNF-α gene expression and levels of TNF-α, IL-1β, and IL-12 secretion. Our data indicate that GLIS activates the immune system by modulating cytokine production.