NK cells attack cells lacking MHC class I, yet MHC class I-deficient mice have normal numbers of NK cells with intact, albeit diminished, functions. Moreover, wild-type NK cells are tolerant of MHC class I-deficient cells in mixed bone marrow chimeras. In this study, we investigated how the absence of MHC class I affects NK cells. NK cells from β 2-microglobulin-deficient (B2m−/−) and wild-type mice exhibit similar phenotypic and functional characteristics. Both B2m−/− and wild-type Ly49H+ NK cells proliferated robustly and produced IFN-γ after infection with mouse CMV. NK cells in mixed wild-type: B2m−/− chimeric mice were initially tolerant of MHC class I-deficient host cells. However, this tolerance was gradually lost over time and after mouse CMV infection was rapidly broken, with a pronounced rejection of host B2m−/− hematopoietic cells. Thus, although NK cells can be held in check against “missing self,” acute inflammation driven by infection can rapidly break established self-tolerance.