To investigate if transporter associated with antigen processing (TAP)-1 is required for CD8(+) T cell-mediated control of Toxoplasma gondii in vivo, we compared the resistance of TAP-1(-/-), CD8(-/-), and wild-type (WT) mice to infection with the parasite. Unexpectedly, TAP-1(-/-) mice displayed greater susceptibility than CD8(-/-), beta(2)-microglobulin(-/-) (beta(2)m(-/-)), or WT mice to infection with an avirulent parasite strain. The decreased resistance of the TAP-1(-/-) mice correlated with a reduction in the frequency of activated (CD62L(low) CD44(hi)) and interferon (IFN)-gamma-producing CD4(+) T cells. Interestingly, infected TAP-1(-/-) mice also showed reduced numbers of IFN-gamma-producing natural killer (NK) cells relative to WT, CD8(-/-), or beta(2)m(-/-) mice, and after NK cell depletion both CD8(-/-) and WT mice succumbed to infection with the same kinetics as TAP-1(-/-) animals and displayed impaired CD4(+) T cell IFN-gamma responses. Moreover, adoptive transfer of NK cells obtained from IFN-gamma(+/+), but not IFN-gamma(-/-), animals restored the CD4(+) T cell response of infected TAP-1(-/-) mice to normal levels. These results reveal a role for TAP-1 in the induction of IFN-gamma-producing NK cells and demonstrate that NK cell licensing can influence host resistance to infection through its effect on cytokine production in addition to its role in cytotoxicity.