We demonstrate herein evidence that IL-12-activated alpha beta T cells with intermediate TCR (NK1+ TCRint cells) in the liver inhibit metastases in the lung as well as in the liver metastases of i-v. injected tumors. IL-12 administration enhanced NK1 expression of NK1+ TCRint cells (NK1high) and increased CD4 weakly positive (CD4low) TCRint cells, while both CD4+ TCRint cells and double-negative TCRint cells were proportionally diminished. Accordingly, the major parts of NK1high TCRint cells are CD4low cells, and most of these cells are V beta 8+ cells. The cytotoxic assays of IL-12-stimulated hepatic mononuclear cells after treatment with respective Abs and complement in vitro and after sorting revealed that CD4low NK1high TCRint cells are cytotoxic effectors. When IL-12-stimulated hepatic mononuclear cells (but not splenocytes) were transferred into tumor-preinjected mice, EL-4 cell metastases in the liver as well as 3LL cell metastases in the lung were inhibited. The antimetastasis of hepatic mononuclear cells transfer was abrogated by the depletion of NK1+ cells, CD3+ cells, or CD4+ cells but not CD8+ cells before transfer. Moreover, transfer of these cells of nude mice into tumor-preinjected mice also inhibited metastases in both organs. Although NK1+ TCRint cells are nearly absent in the hepatic vein blood, a significant proportion of NK1high TCRint cells appeared by IL-12 administration. These results demonstrate that IL-12-stimulated liver NK1high TCRint cells, including extrathymic ones, are major effectors against tumor metastasis and suggest that the cells migrate and inhibit lung metastases.