NK cells and T cells express killer cell inhibitory receptors (KIR) recognizing polymorphic MHC class I molecules. Although prior studies have established that MHC class I can protect normal and transformed hematopoietic cells from NK cell lysis, the role of MHC class I on the recognition of solid tumors has been controversial. In this study, we investigated whether interactions of KIR with their ligands on melanoma tumor cells could inhibit tumor cell lysis by NK and gamma delta T cell clones. Ligation of the NK cell receptor KIR3DL1 by HLA-Bw4 allotypes resulted in inhibition of cytotoxicity against HLA-B*4403-transfected melanomas as well as against melanomas endogenously expressing HLA-Bw4 allotypes. Similarly, interactions of KIR2DL2 or KIR2DL3 (KIR2DL2/3) with HLA-Cw3-related allotypes on melanomas resulted in decreased tumor cell lysis. We also investigated whether signaling via KIR affected melanoma recognition by CTL. Introduction of KIR3DL1 molecules into HLA-A*0201-restricted gp100-specific CTL resulted in inhibition of lysis of gp100+ melanomas co-expressing HLA-A*0201 and HLA-Bw4 allotypes. These results suggest that disrupting interactions of KIR with their ligands on tumor cells in vivo may enhance antitumor responses mediated by both innate and adaptive immune effector cells.