Interferon is important for anti-viral defense of the host. The E2, NS3/4A, and NS5A proteins of hepatitis C virus (HCV) have recently been reported to confront anti-viral action induced by interferon. However, roles of the individual HCV proteins in anti-interferon action are still not well understood. We have isolated an HCV strain, HCV-K, from a patient with acute hepatitis. Nucleotide sequencing of the entire genomic DNA of HCV-K revealed that the isolate belongs to the genotype 1b, which is generally resistant to interferon therapy. In the present study, we expressed individual HCV-K proteins in mammalian cells and investigated effects of the proteins on interferon signal transduction. The results showed that the core, E1, NS4A, and NS4B proteins suppressed activation of interferon stimulation responsive element (ISRE) and gamma activation sequence (GAS) reporters. These results suggest that multiple HCV proteins have a function in suppression of the anti-viral effect by interferon and may indicate a novel role of E1 and NS4B proteins in interferon antagonism.