Until 1988 the putative agent of non-A/non-B (NANB) hepatitis had not been found. Research workers of the Chiron Corporation (California, USA) then identified, by "blind expression cloning", polypeptides which specifically bound antibodies present in sera of NANB-patients. A fusion polypeptide (C-100) was expressed in yeast. With the C-100 antigen prototype RIA and ELISA antibody tests were developed. Subsequently more polypeptides (C-200, C33c, C22) of the HCV-genome were added to the test system, resulting in second generation anti-HCV tests with increased sensitivity. For confirmation of HCV ELISA reactive samples, recombinant immunoblot (RIBA-2, Ortho; Innolia, Innogenetics) and dot immunoblot assays (Matrix, Abbott) were developed. Detection of HCV antigens has been hampered by the low virus titres in serum and the absence of free circulating viral antigen(s). However, with cDNA-PCR, applying primers of the highly (> 93% nucleotide homology) conserved 5' untranslated (5'UTR) region of the HCV genome, HCV-RNA in serum as well as in liver tissue can be detected. cDNA-PCR, although not yet commercially available, is useful to confirm HCV-viremia in patients. When chronic hepatitic C patients are treated with anti-viral drugs, the disappearance of HCV-RNA, as detected by PCR, is a measure of therapy response.