NK cells monitor expression of MHC class I by inhibitory receptors and preferentially kill cells that lose or down-regulate MHC class I expression. One possible mechanism by which tumor cells evade NK cell killing is continued expression of appropriate MHC class I ligands to engage inhibitory receptors on NK cells. We show here that small-mol.-wt blockers against the mouse inhibitory NK cell receptor Ly49A enhance NK cell killing of such tumor cells. We identified Ly49A-binding peptides by selecting phages with the capacity to bind recombinant Ly49A expressed in Escherichia coli from a phage display random peptide library. The Ly49A-binding peptides could also bind Ly49A expressed on mammalian cells. Importantly, the Ly49A-binding peptides blocked Ly49A recognition of its MHC class I ligands H-2Dd and H-2Dk. Moreover, blockade of Ly49A by the peptides enhanced cytotoxicity of Ly49A+ NK cells towards H-2Dd-expressing tumor cells. These results clearly indicate effectiveness of small-mol.-wt blockers of inhibitory NK cell receptors in enhancing NK cell-mediated killing of tumor cells that are otherwise resistant because of MHC class I expression.