T-cell activation results from productive T-cell receptor (TCR) engagement by a cognate peptide-MHC (pMHC) complex on the antigen presenting cell (APC) surface, a process leading to the polarization of the T-cell secretory machinery toward the APC interface. We have previously shown that the half-life of the TCR/pMHC interaction and the density of pMHC on the APC are two parameters determining T-cell activation. However, whether the half-life of the TCR/pMHC interaction can modulate the efficiency of T-cell secretory machinery polarization toward an APC still remains unclear. Here, by using altered peptide ligands conferring different half-lives to the TCR/pMHC interaction, we have tested how this parameter can control T-cell polarization. We observed that only TCR/pMHC interactions with intermediate half-lives can promote the assembly of synapses that lead to T-cell activation. Strikingly, intermediate half-life interactions can be competed out by short half-life interactions, which can efficiently promote T-cell polarization and antagonize T-cell activation that was induced by activating intermediate half-life interactions. However, short TCR/pMHC interactions fail at promoting phosphorylation of signaling molecules at the T-cell-APC contact interface, which are needed for T-cell activation. Our data suggest that although intermediate half-life pMHC ligands promote assembly of activating synapses, this process can be inhibited by short half-life antagonistic pMHC ligands, which promote the assembly of non activating synapses.