NKRP1 receptors were discovered more than 20 years ago, but due to a lack of appropriate reagents, our understanding of them has remained limited. Using a novel panel of mAbs that specifically recognize mouse NKRP1A, D, and F molecules, we report here that NKRP1D expression is limited to a subpopulation of NK cells, but in contrast to Ly49 receptors appears to be expressed in a normal codominant manner. NKRP1D(-) and NKRP1D(+) NK cells are functionally distinct, NKRP1D(+) cells showing reduced expression of various Ly49 receptors, elevated expression of CD94/NKG2 receptors, and higher IFN-gamma secretion and cytotoxicity than NKRP1D(-) cells. Furthermore, NKRP1D(+) NK cells were unable to kill transfected cells expressing high levels of Clr-b molecules, but readily killed MHC class-I-deficient blast cells that express only low levels of Clr-b. NKRP1A and NKRP1F were expressed at low levels on all splenic and bone marrow NK cells, but mAb-induced cross-linking of NKRP1A and NKRP1F caused no significant enhancement or inhibition of NK cell cytotoxicity and no detectable production of IFN-gamma. NKRP1A, D, and F expression could not be detected on NKT cells, all of which express NKRP1C, and although some activated T cells expressed NKRP1C and perhaps low levels of NKRP1A, no significant expression of NKRP1D or F could be detected. NKRP1 molecules expressed on NK cells or transfectants were down-regulated by cross-linking with mAbs or cell surface ligands, and using this phenomenon as a functional assay for NKRP1-ligand interaction revealed that NKRP1F can recognize CLR-x.