The glycolipid alpha -galactosylceramide (alpha -GalCer), which is presented by CD1d and specifically activates Valpha 14 NKT cells, exerts a potent anti-metastatic effect when administered in vivo. In this study, we demonstrated that alpha -GalCer administration led to rapid elimination of NKT cells by apoptosis in the liver and spleen, after they produced IFN-gamma and IL-4. In contrast, a more prolonged secretion of IFN-gamma was observed by liver and splenic NK cells after alpha -GalCer administration. Cytotoxic activity of liver mononuclear cells was not augmented 3h after alpha -GalCer administration, but was increased at 24 h when NKT cells were mostly depleted. The alpha -GalCer-induced cytotoxic activity was abolished in IFN-gamma -deficient and NK cell-depleted mice as well as CD1-deficient mice, suggesting that the alpha -Galcer-induced cytotoxicity was mainly mediated by IFN-gamma -activated NK cells. While the alpha -GalCer-induced cytotoxicity in vitro was mostly perforin dependent, anti-metastatic effect of alpha -GalCer was impaired in NK cell-depleted or IFN-gamma -deficient mice but not in perforin-deficient mice. Collectively, these results indicated that the anti-metastatic effect of alpha -GalCer is mainly mediated by NK cells, which are activated secondarily by IFN-gamma produced by alpha -GalCer-activated NKT cells, in a perforin-independent manner.