This study examined the effects of red ginseng acidic polysaccharide (RGAP) on macrophage-mediated cytotoxicity towards murine melanoma B16 cells. RGAP alone had no effect on killing of tumor cells. RGAP treatment increased the production of interleukin-1 (IL-1), IL-6, and nitric oxide (NO) by macrophages, whereas tumor necrosis factor (TNF-alpha) and reactive oxygen species (ROS) production were not changed by RGAP. However, treatment of macrophages with a combination of RGAP and recombinant interferon-gamma (rIFN-gamma) enhanced killing of tumor cells. In addition, the combination treatment showed marked cooperative induction of IL-1, IL-6, TNF-alpha, and NO production. Electrophoretic mobility shift assay analysis revealed that treatment of macrophages with RGAP plus rIFN-gamma induced the activation of the nuclear factor-kappa B (NF-kappaB) transcription factor. In agreement with this, the combination treatment resulted in increased NF-kappaB-p65 expression. The present results demonstrate synergistic effects on macrophage function of RGAP in combination with rIFN-gamma, and suggest that NF-kappaB plays an important role in mediating these effects. These data also support the development of clinical studies of this combination.