It is widely believed that self-tolerance of natural killer (NK) cells occurs because each NK cell expresses at least one inhibitory receptor specific for a host major histocompatibility complex (MHC) class I molecule. Here we report that some NK cells lack all known self-MHC-specific inhibitory receptors, yet are nevertheless self-tolerant. These NK cells exhibit a normal cell surface phenotype and some functional activity. However, they respond poorly to class I-deficient normal cells, tumor cells, or cross-linking of stimulatory receptors, suggesting that self-tolerance is established by dampening stimulatory signaling. Thus, self-tolerance of NK cells in normal animals can occur independently of MHC-mediated inhibition, and hyporesponsiveness plays a role in self-tolerance of NK cells, as also proposed for B and T cells.