Chronic hepatitis C virus (HCV) infection is a worldwide health problem causing serious complications, such as liver cirrhosis and hepatoma. Alpha interferon (IFN-␣) or its polyethylene glycol-modified form combined with ribavirin is the only recommended therapy. However, an alternative therapy is needed due to the unsatisfactory cure rate of the IFN-based therapy. Using a modified reporter assay based on the HCV subgenomic-replicon system, we found that sodium stibogluconate (SSG), a compound used for leishmania treatment, suppressed HCV replication. We have previously reported that SSG is effective at inhibiting HCV replication in a cell line permissive for HCV infection/replication and in an ex vivo assay using fresh human liver slices obtained from patients infected with HCV (26). In this study, we show that the SSG 50% inhibitory dose for HCV replication is 0.2 to 0.3 mg/ml (equivalent to 345 to 517 M of Sb) in the HCV subgenomicreplicon system. We also found that SSG and IFN-␣ exert a strong synergistic anti-HCV effect in both the traditional isobologram analysis and the median effect principle (CalcuSyn analysis). The combination of SSG and IFN-␣ could sustain the antiviral response better than SSG or IFN-␣ alone. The results suggest that SSG may be a good drug candidate for use in combination with other therapeutics, such as IFN-␣ and ribavirin, to treat HCV infection.Hepatitis C virus (HCV) causes a serious health problem worldwide (23). According to an estimate from the World Health Organization, approximately 3% of the world's population is infected by HCV (24). Although acute HCV infection is commonly associated with mild symptoms or is sometimes asymptomatic, most patients acquire a persistent infection that leads to severe chronic liver diseases, such as cirrhosis and hepatocellular carcinoma (13). Combination of ribavirin with alpha interferon (IFN-␣) (or its polyethylene glycol-modified form) is the only recommended therapy. However, this treatment achieves only an approximate 50% response rate to HCV genotype 1, the most prevalent genotype in HCV infections. Moreover, IFN-␣ and ribavirin combination therapy is expensive and often causes severe side effects (18). Given the high prevalence and severe clinical sequelae of HCV infection, there is an urgent need for the discovery and development of novel agents for HCV therapy.HCV represents the only genus (Hepacivirus) of the Flaviviridae virus family. HCV contains a single-stranded, positivesense RNA genome of 9.6 kb, which encodes a unique polyprotein of approximately 3,000 amino acids (5). The polyprotein is co-and posttranslationally processed by cellular and viral proteases to produce the mature structural and nonstructural viral proteins. The order of these proteins is NH 2 -C-E1-E2-P7-NS2-NS3-NS4A-NS4B-NS5A-NS5B-COOH. Some HCV-encoded proteins can also be produced from alternate reading frames through ribosomal frameshifting (22, 25). One major obstacle impeding the development of HCV therapeutic strategies is the lack of a reproducible in vi...