A lambda gt11-random-primed-cDNA clone specific for chronic hepatitis C was isolated from pooled serum presumably infected by hepatitis C virus. The translation product of the clone detect 50% of patients with chronic hepatitis C in 4 test panels but none of the patients with acute hepatitis C, other liver diseases or normal controls was positive for the peptide. The nucleotide sequence of the cDNA clone, the size of which is 66 bp, has no homology to the complete sequences of known human viruses such as adenovirus, coxsackievirus, rhinovirus, immunodeficiency virus type 1, Epstein-Barr virus, polioma virus, poliovirus, papilloma virus, parvovirus, papovavirus, varicella-zoster virus, yellow fever virus, endogenous retrovirus, T-cell lymphotropic virus types I, II, and III Japanese encephalitis virus, and hepatitis A, B, and D viruses. Probably only one or two epitopes are present on the molecule encoded by the clone as the peptide consists of only 22 amino acid residues.