Recently, the Z27 mAb was shown to recognize the NK cell-activating receptor KIR3DS1, and several genetic studies suggest that the most probable ligands of KIR3DS1 are HLA class I molecules with the Bw4 motif. Despite these findings, the attempts to establish a functional interaction between KIR3DS1 and its potential ligand have been unsuccessful. Here, we study the proliferation and cytotoxicity of KIR3DS1 ؉ NK cells, compared with KIR3DL1 ؉ NK cells, according to the Bw4 ؉ or Bw4 ؊ allogeneic environment. Our results show for the first time that KIR3DS1 expression on NK cells can be induced after exposure to stimulator cells (221, K562, EBV-B cell lines, and B cells), polyinosinic-polycytidylic acid, IL-15, or IL-2. Furthermore, whereas KIR3DL1 ؉ NK cell proliferation and cytotoxicity were inhibited in a Bw4 ؉ but not a Bw4 ؊ context, KIR3DS1 ؉ NK cell functions were not influenced by the presence of Bw4 on target cells. Nevertheless, despite the absence of demonstrated regulation of KIR3DS1 ؉ NK cell functions by HLA-Bw4 molecules, we found a higher KIR3DS1 ؉ NK cell frequency and higher levels of KIR3DS1 expression in Bw4 ؉ compared with Bw4 ؊ individuals. Altogether, these results suggest that KIR3DS1 does not recognize HLA-Bw4 molecules in a physiological context, and they highlight the induced expression of KIR3DS1 observed on stimulated NK cells and the higher frequency of KIR3DS1 ؉ NK cells in Bw4 ؉ individuals. Because a protective KIR3DS1-Bw4 association has been reported in viral infections, our results further the understanding of the role of KIR3DS1 ؉ NK cells in controlling viral infections.