It is estimated that more than 170 million people are infected with hepatitis C virus (HCV) worldwide(1,2). Clinical trials have demonstrated that, for the first time in human history, the potential exists to eradicate a chronic viral disease using combination therapies that contain only direct-acting antiviral agents(3). HCV nonstructural protein 5A (NS5A) is a multifunctional protein required for several stages of the virus replication cycle(4). NS5A replication complex inhibitors, exemplified by daclatasvir (DCV; also known as BMS-790052 and Daklinza), belong to the most potent class of direct-acting anti-HCV agents described so far, with in vitro activity in the picomolar (pM) to low nanomolar (nM) range. The potency observed in vitro has translated into clinical efficacy, with HCV RNA declining by similar to 3-4 log(10) in infected patients after administration of single oral doses of DCV. Understanding the exceptional potency of DCV was a key objective of this study. Here we show that although DCV and an NS5A inhibitor analogue (Syn-395) are inactive against certain NS5A resistance variants, combinations of the pair enhance DCV potency by >1,000-fold, restoring activity to the pM range. This synergistic effect was validated in vivo using an HCV-infected chimaeric mouse model. The cooperative interaction of a pair of compounds suggests that NS5A protein molecules communicate with each other: one inhibitor binds to resistant NS5A, causing a conformational change that is transmitted to adjacent NS5As, resensitizing resistant NS5A so that the second inhibitor can act to restore inhibition. This unprecedented synergistic anti-HCV activity also enhances the resistance barrier of DCV, providing additional options for HCV combination therapy and new insight into the role of NS5A in the HCV replication cycle.