Lectin-like transcript-1 (LLT1) (also named osteoclast inhibitory lectin or CLEC2D) is a ligand for the human NKR-P1A (CD161) receptor, present on NK cells and T cells. To further understand the physiological relevance of this interaction, we developed mAbs against LLT1, characterized the expression pattern of LLT1, and explored the functional consequence of LLT1 engagement of the NKR-P1A receptor on NK cells and T cells. LLT1 is expressed on TLR-activated plasmacytoid dendritic, TLR-activated monocyte-derived dendritic cells, and on B cells stimulated through TLR9, surface Ig, or CD40. Interactions between NKR-P1A on NK cells and LLT1 on target cells inhibit NK cell-mediated cytotoxicity and cytokine production and can inhibit TNF-alpha production by TCR-activated NKR-P1A(+) CD8(+) T cells. In contrast, NKR-P1A failed to inhibit or augment the TCR-dependent activation of NKR-P1A-bearing CD4(+) T cells. Expression of LLT1 on activated dendritic cells and B cells suggests that it might regulate the cross-talk between NK cells and APCs.