Paired receptors that consist of highly related activating and inhibitory receptors are widely involved in the regulation of the immune system. Here, we report a mouse orthologue of the human activating paired immunoglobulin-like type 2 receptor (PILR) beta, which was cloned from a cDNA library of natural killer (NK) cells based on its ability to associate with the DAP12 signaling adaptor protein. The activating PILRbeta was expressed not only on NK cells but also on dendritic cells and macrophages. Furthermore, we have identified a novel CD99-like molecule as a ligand for the activating PILRbeta and inhibitory PILRalpha receptors. Transcripts of PILR ligand are present in many tissues, including some T cell lines. Cells expressing the PILR ligand specifically activated NK cells and dendritic cells that express the activating PILRbeta. Our findings reveal a new regulatory mechanism of innate immunity by PILR and its CD99-like ligand.