A Chinese hamster ovary cell line was established which abundantly expresses the second envelope protein (E2) of hepatitis C virus under the control of an exogenous promoter. The expressed E2 protein was found to be a glycoprotein of 58 kDa by immunoprecipitation with sera from patients that had chronic hepatitis C. Using this cell line as antigen in immunofluorescence tests, as high as 93% of patients with non-A non-B hepatitis had antibodies against E2 protein. In Western blots using SDS-denatured E2 protein, however, the detectability of the antibody was drastically reduced to 30%. Immunoprecipitation assays and ELISA, using both native and denatured E2 protein, revealed that antibodies to E2 protein were present in most of the chronic hepatitis C patients and that they reacted only to the native forms.