Depletion of both natural killer 1.1⁺ (NK1⁺) intermediate αβ T‐cell receptor (int T) cells and NK cells by in vivo treatment with anti‐NK1 antibody greatly increased hepatic metastases of intravenously injected EL4 cells as well as pulmonary metastases of 3LL cells in C57BL/6 mice. However, depletion of NK cells alone by anti‐asialo GM1 (AGM1) antibody treatment did not increase the metastases in either organ. Interleukin‐12 (IL‐12) administration into mice induced strong cytotoxicities of NK cell‐depleted liver and lung mononuclear cells (MNC) comparable to those without NK‐cell depletion and inhibited metastases in either organ. In contrast, in both NK cell‐ and NK1⁺ int T‐cell‐depleted mice, IL‐12 could not induce cytotoxic activity of liver and lung MNC and metastases in both organs increased with or without IL‐12 treatment. These results confirmed the fact that NK1⁺ int T cells are more potent antitumour effectors than NK cells against experimental haematogenous tumour metastases.