NK-T-B antigen (NTBA) is a CD2 family member that functions as a coreceptor in human NK cell activation. Several receptor/ligand interactions occur between different members of this molecular family. In this study, in order to identify the natural ligand of NTBA, we produced a chimeric protein formed by the NTBA extracellular region fused with the Fc portion of human IgG1 (termed NTBA-Fc*). NTBA-Fc* specifically binds to NTBA cell transfectants but not to cells transfected with other CD2 family members including CD2, CD48, CD84, CD150, CD229, and CD244. Moreover, NTBA-Fc* also binds to NTBA+ but not to NTBA– T cell lines. Enzyme-linked immunosorbent assays, plasmon resonance analysis, as well as NTBA-Fc*-mediated down-regulation of NTBA surface expression further confirmed the occurrence of NTBA/NTBA homophilic interaction. Functionally, in NK cells, NTBA-Fc* promoted a strong production of IFN-γ and TNF-α. Moreover, NTBA-transfected targets displayed increased susceptibility to NK-mediated killing as compared to untransfected cells and this effect was specifically inhibited by anti-NTBA mAb. Altogether our data indicate that NTBA is characterized by self recognition.