Having constructed the serological diagnostics of Hepatitis A and B viruses there remained non identified agent(s): named "Non-A, non-B". The non-A, non-B infecting parenterally is a Flavivirus, with lipoid envelope and a diameter of 30-60 nm, as stated very early. After having known the simple filament RNS genome, named as Hepatitis C. In the same time it was tried to isolate the antigen and antibody from ill people, without success. There were not usable the surrogate markers too (anti-HBc and ALT). One part of the synthetized polypeptides from little fragments of C virus genome was apt to antibody-detection. In the acute state there was 20-40, in the chronic state 60-80% anti-HCV positive during the illness. This seropositivity is not equal of healing, neither the potency to infect. The infectivity is showed by the nucleic acid's presence (of the virus) in the blood (PCR), the perfect healing by the neutralizing antibody.