Significance Natural killer (NK) cell function is critically regulated by inhibitory receptors for MHC class I. This work shows that peptides derived from both host and viruses can engage CD94 in the absence of a signalling partner and augment inhibition of NK cells expressing NK cell receptor gene 2A. This result establishes CD94 as a receptor that binds HLA-E in a peptide-dependent fashion. It also demonstrates that NK cells expressing inhibitory receptors from the killer cell immunoglobulin-like receptor or C-type lectin-like receptor families respond with different stoichiometries to changes in the levels of cell-surface MHC class I. Thus these two receptor families may have distinct but complementary functions in recognizing changes in MHC class I.