We previously reported that killer cell immunoglobulin-like receptors (KIR) could be down-regulated from the surface of T cells. Here, we show that KIR down-regulation is also induced on the surface of natural killer (NK) cells upon ligand binding. Common down-regulation characteristics are found on these two cell types: a slow kinetics and a phenomenon observed for long inhibitory forms only. Importantly, KIR down-regulation on NK cells is associated with a down-regulation of activating receptors (CD16, CD2 and 2B4) as well as with a lack of cell responsiveness (antibody-dependent and natural killing activities). This unresponsive state was not observed for MHC-restricted T cells. Our data implicate that, in addition to prevention of the immediate target cell lysis, KIR-MHC class I interactions may also regulate the subsequent NK cell cytotoxic activity. This observation opens new perspectives in the understanding of NK cell regulation.