The class Ib molecule Qa-1 b binds the class Ia leader peptide, Qdm, which reacts with CD94/NKG2R on NK cells. We have generated a gene that encodes the Qdm peptide covalently attached to β 2-microglobulin (β 2 M) by a flexible linker (Qa-1 determinant modifier (Qdm)-β 2 M). When this construct is expressed in TAP-2− or β 2 M− cells, it allows for the expression of a Qdm-β 2 M protein that associates with Qa-1 b to generate the Qdm epitope, as detected by Qdm/Qa-1 b-specific CTL. To test the biological significance of expression of this engineered molecule, we injected TAP-2− RMAS-Qdm-β 2 M cells into C57BL/6 mice and measured their NK cell-mediated clearance from the lungs at 2 h. RMAS cells transfected with Qdm-β 2 M were resistant to lung clearance, similar to RMA cells or RMAS cells in anti-asialo-GM 1-treated mice, while untransfected or β 2 M-transfected RMAS cells were rapidly cleared. Further, pulsing RMAS cells with either Qdm, a K b-, or D b-binding peptide showed equivalent protection from clearance, indicating that a single class Ia or Ib molecule can afford complete protection from NK cells in this system. In contrast, injection of RMAS cells into DBA/2 animals, which express low levels of receptors for Qdm/Qa-1 b, resulted in protection from lung clearance if pulsed with a K b-or D b-binding peptide, but not the Qa-1 b-binding peptide, Qdm.