Natural killer (NK) cells play an important role in immune response by producing interferon gamma (IFN-gamma) as well as exhibiting cytotoxic function. IFN-gamma produced by NK cells has been suggested to be involved in differentiation of T helper cells. On the other hand, the NKR-P1 molecule was recently identified as one of the important NK cell receptors, and it recognizes certain kinds of oligosaccharides on target cells and triggers NK cells for cytotoxicity. In the present study, we found that NK cells produce great amounts of IFN-gamma upon cross-linking of the NKR-P1 molecule. In contrast, stimulation of NK cells with IL-2 induced proliferation without producing IFN-gamma. Similar to NK cells, NK1.1+ T cells also produced IFN-gamma upon NKR-P1 cross-linking. NK1.1+ T cells produced IFN-gamma but not interleukin 4 (IL-4) upon NKR-P1 cross-linking, whereas they secreted both IFN-gamma and IL-4 upon T cell receptor cross-linking. These results indicate that NKR-P1 is a receptor molecule on NK and NK1.1+ T cells that induces not only cytotoxicity but also IFN-gamma production. Our findings provide a new pathway for IFN-gamma production by NK and NK1.1+ T cells through NKR-P1 molecules; it may be essential for immune regulation.