The development of causal therapies depends on the availability of systems to determine the inhibitory capacity of a compound. As viruses are obligate intracellular parasites, the efficacy of an antiviral drug is usually evaluated in a cell-culture system. Unfortunately, the hepatitis C virus, the principal causative agent of acute and chronic liver disease, cannot be propagated efficiently in the laboratory. However, the recent development of a replicon system opens up an encouraging possibility for drug discovery.