A 'blind' recombinant immunoscreening approach, of general application to studies of infectious diseases, was used to clone and identify the genome of the previously uncharacterized non-A, non-B hepatitis (NANB) virus. This agent is a positive-stranded RNA virus that appears to be distantly related to the flaviviridae family. A recombinant viral antigen (C100-3) was used to develop a capture assay for circulating antibody. Data obtained using this assay indicate that this agent, termed the hepatitis C virus (HCV), is the major cause of post-transfusion, community-acquired and cryptogenic, NANB. Anti-C100-3 antibody appears to be directed towards dominant, non-structural viral epitopes. It is a non-neutralising antibody that develops generally late in infection and is a particularly good marker of chronic, persistent viraemia. Many asymptomatic but infectious blood donors can now be detected using this antibody assay. HCV is associated with the development of hepatocellular carcinoma and possibly, other liver diseases.