Panax ginseng C.A. Meyer has been traditionally used for the prevention and treatment of various chronic diseases and infections. Ginseng marc is a fibrous and insoluble by-product remaining after the extraction process of ginseng. In this research an extrusion process was employed to disintegrate the insoluble ginseng marc structure, and water-soluble ginseng marc polysaccharide (GMP) was isolated. GMP was examined for immunomodulatory effects in murine peritoneal macrophages. GMP significantly increased the lysosomal phosphatase activity and the phagocytic index of peritoneal macrophages (P<.05). The peritoneal macrophages treated with GMP also produced significantly more H(2)O(2) and nitrite than the control without GMP treatment (P<.05). In addition, GMP (100 microg/mL) significantly increased the cell viability of peritoneal macrophages (P<.05). These results suggest that GMP is an effective nonspecific immunomodulatory agent, and its immunostimulating effects may be due to its ability to stimulate the production of reactive oxygen intermediates.