We previously reported that an acidic polysaccharide from Panax ginseng named ginsan inhibits the incidence of benzo[a]pyrene-induced autochthonous lung tumors in mice. To elucidate the mechanism of antineoplastic activity, ginsan was tested for its ability to generate LAK cells and to produce cytokines. Spleen cells became cytotoxic to a wide range of tumor cells after 5 days of culture with ginsan in a non-major histocompatibility restricted manner and the activity of ginsan was 12 times higher than that of lentinan. The generation of killer cells by rIL-2 was neutralized only in the presence of anti-IL-2, whereas by ginsan it was neutralized in the presence of anti-IL-2 as well as anti-IFN gamma, or anti-IL-1 alpha. It was confirmed that ginsan induces the expression of mRNA for IL-2, IFN gamma, IL-1 alpha, and GM-CSF. Depletion of AsGM1+ cells from spleen cells reduced the generation of LAK by rIL-2. In contrast, depletion of AsGM1+ as well as Thy1+ cells, CD4+ cells, or DC8+ cells reduced the generation of LAK cells by ginsan. The serologic phenotype of rIL-2 induced LAK cells was CD8- cells, whereas the ginsan induced LAK cells, were CD8+ cells. Ginsan synergized with rIL-2 to generate LAK cells (2.0-15 fold) and the most dramatic synergy was seen at rIL-2 concentrations below 3 U/ml. Ginsan alone inhibited pulmonary metastasis of B16-F10 melanoma cells and enhanced the inhibition of lung colonies by rIL-2. These findings demonstrate that ginsan generates LAK cells from both NK and T cells through endogeneously produced multiple cytokines. This property may contribute to its effectiveness in the immunoprevention and immunotherapy of cancer.