The prevention of non-A, non-B hepatitis (NANB-PTH) is of the utmost importance in the blood bank setting, since the estimated attack rate amounts to 0.1-1.0% in Germany and greater than or equal to 2% in the USA. The value of 'surrogate markers', anti-HBc and ALT, remains a matter of controversy. In early 1988, a small (60 nm), single-stranded RNA-virus (toga- or flaviviridae), probably causing both sporadic and parentally transmitted NANBH was isolated, partially cloned by a lambda gt 11 expression vector and named hepatitis C virus (HCV) by Choo, Houghton et al. The HCV fusion protein was used to detect specific antibodies by EIA in patients with NANBH. Preliminary data obtained with this EIA (Chiron Corp., Emeryville, Calif., USA) in US blood donors yielded a seroprevalence rate of 0.7%. of 8.3% in donors with a history of hepatitis, of 5.1% in donors with ALT levels greater than or equal to 65 IU, and of 45% in NANB patients. Investigations of critical 'blood donor/transfusion recipient' pairs revealed a 73% correlation of concordant seropositiveness. The evaluation of an improved HCV antibody EIA (Ortho Diagnostic Systems, Neckargemünd, FRG) will be the subject of a German multicentre pilot study scheduled for 1989. The introduction of this HCV test should drastically reduce the most serious infectious risk of homologous blood transfusion. Efforts are being made to develop an HCV vaccine.