Killer cell immunoglobulin-like receptors (KIRs) are transmembrane glycoproteins expressed by natural killer (NK) cells. Binding of KIR3DS1 to its recently discovered ligand, HLA-F, activates NK cells and has been associated with resolution of hepatitis C virus (HCV) infection. We investigated the mechanisms by which KIR3DS1 contributes to the antiviral immune response. Using cell culture systems, mice with humanized livers , and primary liver tissue from HCV-infected individuals, we found that the KIR3DS1 ligand HLA-F is upregulated on HCV-infected cells, and that interactions between KIR3DS1 and HLA-F contribute to NK cell-mediated control of HCV.