THE molecular basis of target cell recognition by CD3⁻ natural killer (NK) cells is poorly understood, despite the ability of NK cells to lyse specific tumour cells^1,2. In general, target cell major histocompatibility complex (MHC) class I antigen expression correlates with resistance to NK cell-mediated lysis^3–9, possibly because NK cell-surface molecules engage MHC class I antigens and consequently deliver inhibitory signals^3,4. Natural killer cell allospecificity involves the MHC class I peptide-binding cleft¹⁰, and further understanding of this allospecificity should provide insight into the molecular mechanisms of NK cell recognition. The Ly-49 cell surface molecule is expressed by 20% of CD3⁻ NK cells¹¹ in C57BL/6 mice (H–2^b). Here we show that C57BL/6-derived, interleukin-2-activated NK cells expressing Ly-49 do not lyse target cells displaying H–2^d or H–2^k despite efficient spontaneous lysis by Ly-49⁻ effector cells. This preferential resistance correlates with expression of target cell MHC class I antigens. Transfection and expression of H–2D^d, but not H–2K^d or H–2L^d, renders a susceptible target (H–2^b) resistant to Ly-49⁺ effector cells. The transfected resistance is abrogated by monoclonal anti-bodies directed against Ly-49 or the α1/α2 domains of H–2D^d, suggesting that Ly-49 specifically interacts with the peptide-binding domains of the MHC class I alloantigen, H–2D^d. Inas-much as Ly-49⁺ effector cells cannot be stimulated to lyse H–2D^d targets, our results indicate that NK cells may possess inhibitory receptors that specifically recognize MHC class I antigens.