We have compared the immune responses of mice immunized either with alpha-galactosylceramide (alpha-GalCer), capable of eliciting a CD1-metiated stimulation of V alpha14+ NK T cells, or with lipoarabinomannan (LAM), a glycophospholipid derived from mycobacteria which is known to be presented by CD1b in humans. Within 24 h, alpha-GalCer induces a burst of IFN-gamma secretion in vivo, and recall with antigen in vitro leads to the synthesis of IL-4 and IL-10 in addition to IFN-gamma. Associated with this in vivo cytokine release is a polyclonal activation of splenic B and T cells. CD1-reactive NK T lymphocytes mediate these events, because none of them are observed in alpha-GalCer-immunized CD1-/- mice. LAM immunization fails to promote similar early responses in vivo. Repeated exposure of mice to alpha-GalCer induces splenic T cells to secrete IL-4 and IL-10 but dramatically reduced levels of IFN-gamma. Such a bias in the cytokine balance triggered by NK T cells stimulated with multiple doses of alpha-GalCer suggests that this compound might be useful in the induction of Th2 immune responses and the prevention of chronic inflammatory conditions mediated by Th1 cytokines.