The potential of hepatitis C virus (HCV) to develop antiviral resistance renders phenotypic analysis of viral relapse or breakthrough sequences essential to the clinical evaluation of HCV antivirals. This work describes a transient assay in which clinical NS3/4A sequences are co-expressed in Huh-7 cells with a reporter whose activity is an easily quantifiable measure of protease activity. The utility of the assay was demonstrated in potency evaluations of a novel protease inhibitor against panels of NS3/4A sequences spanning genotypes 1-3. The compound was potent against genotype 1a and 1b protease sequences with sub-nanomolar to low nanomolar EC(50)s, slightly diminished in potency against genotype 2b sequences, but poorly active against genotype 3a sequences. Diverse sequences of the same HCV genotype, however, varied in response to the inhibitor as much as 30-fold, with susceptibility differences not easily attributed to specific amino acid polymorphisms. The results demonstrate the versatility of a novel phenotype assay in the evaluation of a promising new class of NS3/4A inhibitor. The results highlight further the complexity in correlating susceptibility differences with specific sequence polymorphisms, and underscore the value in direct phenotyping of clinical sequences for compound sensitivity. The assay will be useful for monitoring changes in susceptibility due to emergence of resistant virus during clinical studies of protease inhibitors.