A major subset of non-alloreactive NK cells in PVG strain rats is generally low in Ly49 receptors, but expresses the rat NKR-P1B(PVG) receptor (previously termed NKR-P1C). The NKR-P1B(+) NK subset is inhibited by a non-polymorphic target cell ligand, which we have shown here to be a C-type lectin-related molecule (Clr). Clr11 ligates two divergent NKR-P1B alleles as judged by an NFAT-driven reporter assay, and inhibits NK-cell cytotoxicity of NKR-P1B(+) NK cells. Clr11 also interacts with the prototypic NKR-P1A receptor and exerts a stimulatory influence on NK lysis. NKR-P1A and B are encoded by adjacent genes in the proximal part of the NK gene complex and show close sequence homology in their extracellular region. They diverge from another pair, NKR-P1F and -G, which is encoded by a second, distal Nkrp1 gene cluster. NKR-P1F and -G bind an overlapping panel of Clr ligands, but not Clr11. Rat Clr molecules appear to be constitutively expressed by hematopoietic cells; expression in tumor cell lines is more variable. The data show the existence of two phylogenetic groups of NKR-P1 molecules, which demonstrate conservation of ligand-binding properties independent of signaling function.