We utilized gene targeting by homologous recombination to define the role that MEF, a transcriptional activating member of the ETS family of transcription factors, plays in lymphopoiesis. MEF−/− mice have a profound reduction in the number of NK-T and NK cells. Purified MEF−/− NK cells cannot lyse tumor cell targets and secrete only minimal amounts of IFNγ. Perforin protein expression is severely impaired in MEF-deficient NK cells, likely accounting for the lack of tumor cell cytotoxicity. Promoter studies and chromatin immunoprecipitation analyses demonstrate that MEF and not ETS-1 directly regulates transcription of the perforin gene in NK cells. Our results uncover a specific role of MEF in the development and function of NK cells and in innate immunity.