The objective of the current study was to determine the effects of the crude polysaccharide isolated from fruit body of Pleurotus eryngii on mouse splenocytes, B cells, and macrophages in vitro. The crude polysaccharides directly induced the proliferation of spleen cells in a dose-dependent manner and increased IL-6 and IFN-${gamma}$ synthesis. The crude polysaccharides also increased the proliferation of B cells in a dose-dependent manner. The production of immunoglobulin Gl, G2a and IgG3 in the presence of the crude polysaccharides was increased progressively in the culture supernatant. When the crude polysaccharide were used in macrophage cell line (RA W264.7) stimulation, there were marked induction of NO synthesis in a dose-dependent manner and IL-6, TNF- r and GM-CSF synthesis. These results suggest that the crude polysaccharide isolated from fruit body of Pleurotus eryngii seem to act as a potent immunomodulator causing augmentation of immune cell activity, and thus could be used as a biological response modifier having possible therapeutic effects against immunological disorders, without any side effects.