The successful cloning of a non-structural antigen from the genome of what is now designated as the 'hepatitis C virus' (HCV) has transformed an erstwhile diagnosis of exclusion for non-A, non-B hepatitis (NANBH). The assay has been validated against panels of known infectivity for NANBH and sera from haemophiliac patients treated either with virally inactivated or uninactivated factor VIII. The predictive value of the assay is being assessed clinically in prospective studies of post-transfusion hepatitis and by using laboratory techniques such as polymerase chain reaction. While the assay shows good predictability in high-risk subjects, an appreciable number of false-positive results are likely in blood donor populations. Furthermore, the extent of infectivity of seropositive blood donors is still the subject of active research. The prevalence of anti-HCV in blood donors varies from approximately 0.2 to 1.5% around the world, based on repeat reactivity in the Ortho antiglobulin ELISA assay. These rates may be appreciably reduced following supplementary testing with recombinant immunoblot assay (RIBA). Prevalence data in African sera are as yet unreliable, pending assessment by RIBA, presumably because of high levels of IgG interfering with the assay. Presence of anti-HBc or elevated alanine aminotransferase associates to a greater or lesser extent with seropositivity, especially when both surrogate markers are present, but conversely many (unconfirmed) seropositive subjects lack these surrogate markers. An understanding of the modes of transmission of HVC is of obvious importance to transfusion practice. Intravenous drug use is a striking risk factor, but the contribution made by sexual transmission is not so clear.(ABSTRACT TRUNCATED AT 250 WORDS)