Natural killer T (NKT) cells are a T-cell subpopulation known to possess immu-noregulatory functions and recognize CD1d molecules. The majority of NKT cells express an invariant T-cell receptor (TCR) α chain rearrangement (Vα14Jα18 in mice; Vα24Jα18 in humans) and are called type I NKT cells; all other NKT cells are type II. In the current study, we have analyzed the roles for these NKT-cell subsets in the host's innate antitumor response against a murine B-cell lym-phoma model in vivo. In tumor-bearing mice, we found that type I NKT cells conferred protection in a CD1d- dependent manner, whereas type II NKT cells exhibited inhibitory activity. Pro- and anti-inflammatory cytokines secreted by splenocytes from tumor-bearing mice correlated with tumor progression. Myeloid cells (CD11b+Gr1 +) were present in large numbers at the tumor site and in the spleen of tumor-bearing type I NKT-deficient mice, suggesting that antitumor immunosurveillance was inhibited by CD11b+Gr1+ cells. Overall, these data suggest that there are distinct roles for NKT-cell subsets in response to a B-cell lymphoma in vivo, pointing to potential novel targets to be exploited in immuno-therapeutic approaches against blood cancers.