A lymphocyte subpopulation, the V(alpha)14 natural killer T (NKT) cells, expresses both NK1.1 and a single invariant T cell receptor encoded by the V(alpha)14 and J(alpha)281 gene segments, Mice with a deletion of the J(alpha)281 gene segment were found to exclusively lack this subpopulation. The V(alpha)14 NKT cell-deficient mice could no longer mediate the interleukin-12 (IL-12)-induced rejection of tumors. Although the antitumor effect of IL-12 Was thought to be mediated through natural killer cells and T cells, V(alpha)14 NKT cells were found to be an essential target of IL-12, and they mediated their cytotoxicity by an NK-like effector mechanism after activation with IL-12.