In South America, natural products with unknown drug effects are used as folk remedies and for preventive medicine. Among South American natural products, we directed our attention to Propolis, which have been known as medicinal plants, and examined the mechanisms by which these substances affect antioxidant activity, anti-tumor activity and immunoresponse. When the antioxidant activities of Propolis were examined by the DPPH and Rhoudan iron methods, since Propolis contains high levels of flavonoids, it is thought that flavonoids may be responsible for the antioxidant activity in this study. In the examination of immunoenhancement activity, we measured lymphocyte versus polymorphonuclear leukocyte ratios (L/P activity). The number of lymphocytes was significantly increased in groups treated with Proplolis. Specifically, slightly high levels of $IFN-{gamma}$ were measured in mice bearing the S-180 carcinoma, after administration of Propolis. This strongly suggests that cellular immunity is especially activated by treatment with Propolis, because production of $IFN-{alpha}$ is limited to the T cells and NK cells stimulated by mitogen and sensitized antigen. $TNF-{alpha}$ shows a different extent and mechanism of action depending on the target cells. When $TNF-{alpha}$ was measured in mice bearing the S-180 carcinoma, mice treated with Propolis showed slightly higher $TNF-{alpha}$ levels as compared to the control group. This suggests that activated macrophages produce $TNF-{alpha}$ in mice treated with Prapolis, since activated macrophages and lymphocytes are the source of most $TNF-{alpha}$. When anti-tumor action was examined using two kinds of sarcoma (Ehrlich solid carcinoma and Sarcoma-180 carcinoma), tumor-suppressive ratios after treatment with Propolis was 29.1%. When Sarcoma-180 solid carcinoma was used, tumor-suppressive ratios were 62%. Thus, Propolis showed strong anti-tumor activity against two kinds of solid carcinoma. Taken altogether, this strongly suggests that Propolis enhances original functions of macrophages and NK cells, and as a result, secondarily enhances the immune reaction and suppresses tumor growth.