AbstractB7‐H3 belongs to the B7 superfamily, a group of molecules that costimulate or down‐modulate T‐cell responses. Although it was shown that B7‐H3 could inhibit T‐cell responses, several studies – most of them performed in murine systems – found B7‐H3 to act in a costimulatory manner. In this study, we have specifically addressed a potential functional dualism of human B7‐H3 by assessing the effect of this molecule under varying experimental conditions as well as on different T‐cell subsets. We show that B7‐H3 does not costimulate human T cells. In the presence of strong activating signals, B7‐H3 potently and consistently down‐modulated human T‐cell responses. This inhibitory effect was evident when analysing proliferation and cytokine production and affected naïve as well as pre‐activated T cells. Furthermore, we demonstrate that B7‐H3–T‐cell interaction is characterised by an early suppression of IL‐2 and that T‐cell inhibition can be reverted by exogenous IL‐2. Since the triggering receptor expressed on myeloid cells like transcript 2 (TREML2/TLT‐2) has been recently described as costimulatory receptor of murine B7‐H3 we have extensively analysed interaction of human B7‐H3 with TREML2/TLT‐2. In these experiments we found no evidence for such an interaction. Furthermore, our data do not point to a role for murine TREML2 as a receptor for murine B7‐H3.