We found that Aged Garlic Extract (AGE) could be a significant immuno-potentiator, and could exhibit anti-tumor activities through immune modulation. Consequently, AGE stimulated the proliferation of mouse spleen cells and the release of cytokines, such as IL-2, TNF-alpha and IFN-gamma, increased NK activities, and enhanced phagocytosis of peritoneal macrophages. AGE treatment also stimulated the reactivity of lymphocytes in response to cytokines or mitogens. AGE was far superior to PSK in IL-2 induction, but slightly inferior to PSK in nitric oxide induction. AGE, as effectively as PSK (Krestin), significantly inhibited the growth of Sarcoma-180 (allogenic) and LL/2 lung carcinoma (syngenic) cells transplanted into mice. Concomitantly, increases in NK and killer activities of spleen cells were observed in Sarcoma-180 bearing mice treated with AGE. These results strongly suggest that AGE is as effective as PSK, and could serve as a potent biological response modifier on NK cells and T lymphocytes, and subsequently inhibit the growth of transplanted tumors.