BMS-1166 is a potent PD-1/PD-L1 interaction inhibitor. BMS-1166 binds to human PD-L1 and blocks its interaction with PD-1. BMS-1166 alleviates the inhibitory effect of the soluble PD-L1 on the T-cell receptor-mediated activation of T-lymphocytes. Moreover, BMS-1166 was effective in attenuating the inhibitory effect of the cell surface-associated PD-L1.