Co-signal receptors provide crucial activating or attenuating signals for T cells. The B and T lymphocyte attenuator (BTLA/CD272) is a third member of co-inhibitory receptors, which belongs to the CD28 immunoglobulin-superfamily. Using monoclonal antibodies (mAbs) against human BTLA, we show that BTLA is constitutively expressed on most CD4⁺ and CD8⁺ T cells and its expression progressively decreases upon T cell activation. Polarized Th1 and Th2 cells contained both BTLA-positive and BTLA-negative populations, but the extended culture diminished BTLA expression. Cross-linking BTLA with an agonistic mAb inhibited T cell proliferation and the production of the cytokines IFN-γ and IL-10 in response to anti-CD3 stimulation. BTLA-mediated inhibition of T cell activation occurred during both primary CD4⁺ T cell responses and secondary CD4⁺ and CD8⁺ T cell responses, suggesting that BTLA ligation sends a constitutive “off” signal to T cells and thus might play an important role in the maintenance of T cell tolerance.