The acquisition of inhibitory MHC-specific receptors occurs during NK cell differentiation and has been considered important in regulating NK cell responsiveness. NK cell differentiation has been studied on the basis of cell surface phenotype, function, and proliferative capacity. Together with phenotypically immature Mac-1lo NK cells, the mature Mac-1hi NK cell pool can be dissected further into two functionally distinct CD27hi and CD27lo subsets. Two major inhibitory receptors, CD94/NKG2A and Ly-49, are expressed on mouse NK cells. The acquisition of the CD94/NKG2A receptor seems to be an early event, whereas Ly-49 receptor expression is considered a relatively late event during NK cell ontogeny. In this study, we demonstrated a distinct NK cell inhibitory receptor repertoire formation within mature NK cell populations as defined by Mac-1 and CD27. By analyzing mice deficient in MHC class I expression or NKG2D ligand transgenic mice, we have shown that the inhibitory receptor repertoire can be modulated according to the differentiation/maturation status of NK cells, and the receptor acquisition is imprinted at an early stage of NK cell development by MHC class I interactions.