Monoclonal antibodies (MAbs) directed against envelope glycoprotein E1 (gp51-54) of hog cholera virus (HCV) strain Brescia have been shown to recognize four different antigenic domains A, B, C and D. Epitopes of within domain A have mainly been found conserved among HCV strains, whereas epitopes within domains B, C and D are not conserved. We used transiently expressed hybrid E1 genes of HCV strains Brescia and "C" to map the non-conserved epitopes on E1. Epitopes in domains B and C are located within the ultimate N-terminal 104 amino acids. The non-conserved subdomain A3 is most probably located between domains B/C and a hydrophobic region, which is highly conserved between HCV strains Brescia and "C". The conserved subdomains A1 and A2 are probably located in the vicinity and C-terminally of this conserved, hydrophobic region, which is near the centre of the E1 amino acid sequence.