The biological activities and antitumor mechanism of an immunopotentiator, Ge-132, is reviewed herein. Ge-132 exhibited antitumor activity against certain syngeneic and allogeneic experimental tumors. It was shown that T-cells and macrophages were involved when tumor-bearing mice were protected by the compound. This protective effect could be transferred to tumor-bearing mice, not treated with the compound, by a macrophage fraction and serum specimens obtained from Ge-132-treated mice. Interferon gamma (IFN gamma) was detected in the circulation of Ge-132-treated mice and when sera obtained from Ge-132-treated mice were treated with anti-IFN gamma antiserum in vitro, the antitumor activity was abolished. On the other hand, in mice treated with anti-IFN gamma antiserum, Ge-132 did not induce serum IFN and failed to protect against death due to ascites tumor progression. The in vivo administration of monoclonal anti-Thy 1.2 antibody prevented the expression of the antitumor activity of Ge-132. However, serum specimens obtained from Ge-132-treated mice effectively inhibited the tumor growth of T-cell-depleted mice bearing ascites tumors. Since it has been reported that T-lymphocytes produce IFN gamma, this suggested that Ge-132 may first stimulate T-cells to produce IFN gamma in the expression of the observed antitumor efficacy. In addition, sera obtained from Ge-132-treated mice did not show any antitumor activity in mice depleted of macrophage functions. Additionally, passive transfer of macrophages from mice treated with these serum specimens to tumor-bearing mice also resulted in the inhibition of tumor growth. Pretreatment of these serum specimens with anti-IFN gamma antiserum effectively prevented the generation of cytotoxic macrophages. Also, tumor-bearing mice treated exogenously with this antiserum did not differ significantly in survival as compared to controls, despite the administration of Ge-132. Furthermore, the antitumor activity of Ge-132 was detected in NK cell-depleted mice. Therefore, the antitumor mechanism of Ge-132 in the murine ascites tumor system may be expressed as follows: (a) Ge-132 stimulates T-cells to induce IFN gamma when mice are treated orally with the compound, (b) IFN gamma activates macrophages to become cytotoxic, and (c) the cytotoxic macrophages eliminate tumor cells.