Acidic polysaccharide from the ethanol-insoluble and water-soluble fraction of Korean red ginseng was isolated and its immunomodulating activities were investigated. Red ginseng acidic polysaccharide (RGAP), administered intraperitoneally for seven days, induced a high output nitric oxide synthase (NOS2) in vivo. In the presence of interferon y , RGAP induced NOS2 by newly synthesized mRNA in vitro. After the multiple injections of RGAP, spleen cells were proliferated spontaneously in a dose dependent manner, while the concanavalin A (Con A)-induced spleen cell proliferation was inhibited. An addition of NG-monomethyl-L-arginine (NMMA) resumed the Con A-induced spleen cell proliferation. The in vivo pretreatment of aminoguanidine, a specific NOS2 inhibitor, alleviated the RGAP-induced suppressions of antibody plaque-forming cell response to sheep red blood cells as well as cytotoxic T lymphocyte response against P815 tumor cells. RGAP-administration also enhanced the macrophage-mediated tumor killing. In the coculture system of P815 cells with macrophages, proliferation of P815 cells was severely suppressed by the RGAP-activated macrophages. NMMA addition alleviated the suppressive proliferation of P815 cells at the same time as blocked NO accumulation. These results implicate a link between NO production and suppression of the proliferative response. The present study proposes that NO is a pivotal mediator of immunomodulating activities of RGAP.