In 1995 two novel human virae were identified independently, GB virus C and hepatitis G virus, as possible etiological agents for non-A-E viral hepatitis. GBV-C and HGV are two isolates of the same virus. GBV-C/HGV is an RNA virus of app. 9 kb belonging to the Flaviviridae family. GBV-C/HGV is transmitted intravenously, sexually and vertically from mother to child. GBV-C/HGV can be detected in blood by RT-PCR and serological tests based on the envelope 2 protein. Infection by GBV-C/HGV is frequent and 1-10% of blood donors in Western countries have been found positive by PCR. Acute GBV-C/HGV infections are in some cases associated with mild hepatitis. About 10-20% of the infections become chronically persistent but are not associated with chronic hepatitis or other known disease. GBV-C/HGV is not found in hepatocytes and is not infectious in the chimpanzee model, therefore GBV-C/HGV is not a classical hepatitis virus. Detection of GBV-C/HGV by PCR may be indicated in the diagnosis of acute hepatitis.