Our data on 559 serum samples are discussed in the light of the present knowledge on hepatitis C virus (HCV). This newly discovered virus is thought to be the main cause of the bloodborne non-A, non-B hepatitis. HCV is a single-stranded, encapsulated RNS virus. Presently one antibody, anti-HCV, is detectable, which is directed against a nonstructural viral component. This antibody appears only weeks to months after infection or onset of the disease. Within 6-9 months 60-80% of the patients with resolving non-A, non-B hepatitis become positive, with the anti-HCV usually disappearing after 1-5 years. Patients with chronic infections are positive in 70-90% of cases, with the antibody usually persisting over decades. The presence of anti-HCV offers no clue as to whether there is an ongoing or past infection and does not serve to judge infectivity or immunity. 0.34% of the blood donors in Switzerland are anti-HCV positive, with 1.3% in Southern Italy and 0.6-0.8% in Northern Italy, Great Britain, France and Germany. Anti-HCV prevalence is high in hemophiliacs (59-97%), i.v. drug users (48-92%) hemodialysis patients and polytransfused individuals (3-23%), homosexuals and promiscuous heterosexuals (1-40%) as well as in patients with non-B hepatoma (62-80%) or with alcoholic cirrhosis (27-52%). Our own data show anti-HCV in 45% of i.v. drug users, 7% of homosexuals, 13% of patients under hemodialysis or with renal transplants, and in 60% of patients with chronic non-A, non-B hepatitis.