The root of Panax ginseng C. A. Meyer is one of the most popular natural tonics in oriental countries. In this study, we have isolated polysaccharide fraction of Panax ginseng (ginsan) and examined its effect on the function of murine peritoneal macrophages. When macrophages were treated with ginsan, cytotoxic activity against B16 melanoma cells was significantly induced. In addition, the levels of cytokines, including tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), IL-6 and Interferon-gamma (IFN-gamma) were increased and the production of reactive oxygen/nitrogen components such as nitric oxide (NO) and hydrogen peroxide (H2O2) was enhanced. Moreover, phagocytic activity was induced in ginsan-treated macrophages compared to the control. The expression of CD14 and 1-Ab on murine peritoneal macrophages was increased by the treatment with ginsan, while the expression of CD11b was decreased. Taken together, these results suggest that ginsan has an immunopotentiating effects on macrophages and these abilities could be used clinically for the treatment of diseases such as cancer.