The MHC class Ib molecule Qa-1 is the primary ligand for mouse CD94/NKG2A inhibitory receptors expressed on NK cells, in addition to presenting Ags to a subpopulation of T cells. CD94/NKG2A receptors specifically recognize Qa-1 bound to the MHC class Ia leader sequence-derived peptide Qdm. Qdm is the dominant peptide loaded onto Qa-1 under physiological conditions and this peptide has an optimal sequence for binding to Qa-1. Peptide dissociation experiments demonstrated that Qdm dissociates from soluble or cell surface Qa-1 b molecules with a t 1/2 of∼ 1.5 h at 37 C. In comparison, complexes of an optimal peptide (SIINFEKL) bound to the MHC class Ia molecule H-2K b dissociated with a t 1/2 in the range from 11 to 31 h. In contrast to K b, the stability of cell surface Qa-1 b molecules was independent of bound peptides, and several observations suggested that empty cell surface Qa-1 b molecules might be unusually stable. Consistent with the rapid dissociation rate of Qdm from Qa-1 b, cells become susceptible to lysis by CD94/NKG2A+ NK cells under conditions in which new Qa-1 b/Qdm complexes cannot be continuously generated at the cell surface. These results support the hypothesis that Qa-1 has been selected as a specialized MHC molecule that is unable to form highly stable peptide complexes. We propose that the CD94/NKG2A-Qa-1/Qdm recognition system has evolved as a rapid sensor of the integrity of the MHC class I biosynthesis and Ag presentation pathway.