Epitopes recognized by nine monoclonal antibodies (mAbs) on the envelope protein, gp53, of two strains of bovine viral diarrhoea virus (NADL and Oregon C24V) were mapped by competitive binding assays and by the characterization and sequence analyses of mAb neutralization escape mutants. This defined an antigenic domain on gp53 that was shared by many BVDV strains, while other less conserved epitopes were possibly distinct. Sequencing of escape mutant viruses revealed that a cluster of three amino acids in the N-terminal half of gp53 were involved in the main antigenic domain shared by both NADL and Oregon C24V viruses, while an amino acid 31 residues further toward the N-terminus was involved in a second site present only on the NADL strain. Since other amino acids defining these epitopes were located at distant positions within the gp53 protein, it is likely that a major domain [corrected] on gp53 consist of composite, conformation-dependent epitopes.