We investigated the role of human natural killer (NK) cells in the peripheral blood (PB) and liver in controlling breast cancer. The proportion of NK cells among liver mononuclear cells was significantly higher than among PB mononuclear cells. Liver NK cells inductively expressed higher levels of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) than PB NK cells in response to interleukin-2 (IL-2). Liver NK cells displayed higher cytotoxicity against various breast cancer cell lines (MDA-MB231, MDA-MB453, MDA-MB468, and MCF-7) after IL-2 stimulation than did PB NK cells. Anti-HER2 monoclonal antibody (mAb) promoted the cytotoxicity of both the types of NK cells toward HER2-expressing cell lines. All breast cancer cell lines highly expressed death-inducing TRAIL receptors, death receptor 4, but did not express death-inhibitory receptors (DcR1 and DcR2). Both PB and liver NK cell-induced cytotoxicity was inhibited partially by anti-TRAIL mAb and more profoundly by the combination of anti-TRAIL mAb and concanamycin A, indicating that TRAIL and perforin are involved. IL-2-stimulated liver and PB NK cells exhibited upregulated expression of CXCR3, which bind to the chemokines CXCL9, CXCL10, and CXCL11 secreted by breast cancer cells. We also found that IFN-γ promoted the production of CXCL10 from breast cancer cells. The results of this study show that IFN-γ secreted from NK cells likely promotes the production of CXCL10 from breast cancer cells, which in turn accelerates the migration of CXCR3-expressing NK cells into the tumor site. These findings suggest the possibility of a therapeutic approach by either activation of endogenous PB and liver NK cells or adoptive transfer of in vitro-activated autologous NK cells.