Since cell signal transduction plays an important role in disclosing the nature of human diseases, the pathogenesis of viruses may result from the disturbance of intracellular signal cascades caused by viral proteins. Hepatitis C virus (HCV) is a main causative agent of severe human liver disorders worldwide. So far, the mechanisms of HCV pathogenicity remain unclear. Envelope protein 2 (E2) of HCV is thought to be responsible for initiating virus attachment to host cells, which is a prerequisite of HCV infection. We assume that some early events of HCV pathogenic effects may result from the interaction of HCV E2 protein with its cellular receptor (human CD81), which could regulate cell proliferation and differentiation. To test this hypothesis, the effects of HCV E2 protein on MAPK/ERK pathway in Molt-4 and U937 cells with or without human CD81 expression were investigated. The results showed that HCV E2 protein could specifically activate the MAPK/ERK pathway, and such activation was inhibited by monoclonal antibodies against CD81 or HCV E2, serum antibodies from HCV infected patients, and upstream MEK1 inhibitor PD98059. Moreover, HCV E2-driven MAPK/ERK or downstream transcription factor Elk-1 activation was completely blocked in the presence of PD98059. These findings strongly suggest that the regulation of transmembrane signaling by HCV E2 protein via its receptor(s) on host cells might contribute to the development of HCV-related diseases.