Although both envelope glycoproteins of the hepatitis C virus, E1 and E2/NS1, show a high degree of sequence variation, the E1 protein includes a well conserved domain, which may be functionally important. We have analysed the human B cell response to a peptide fragment from amino acid residues 314-330 (EP3) covering the central conserved sequence of this domain. Anti-hepatitis C virus-positive blood donors were screened for anti-EP3 antibodies with an ELISA based on immobilized peptide. Thirty out of 92 (32%) RIBA-confirmed donors displayed a significant antibody response to EP3. From three of these blood donors we established four anti-EP3-producing heterohybridoma cell lines: Ul/F30 and Ul/F31 produced IgM-kappa, whereas Ul/F32 and Ul/F33 secreted the isotypes IgG1-lambda and IgG1-kappa, respectively. Epitope analysis with overlapping nonapeptides suggests the existence of different antigenic determinants within the EP3 fragment. Although both IgG antibodies Ul/F32 and Ul/F33 have dissociation constants to the peptide of approximately 10(-9) M, binding to recombinant E1 protein expressed in COS-7 cells was different. Only Ul/F33 detected envelope protein of approximately 24-35 kD in Western blot. This human MoAb will be useful for further investigations on the hepatitis C virus glycoprotein E1.