Nine monoclonal antibodies (Mabs) to a vaccine strain of infectious bursal disease virus (IBDV) of intermediate virulence were characterized in Western-blot, radioimmunoprecipitation, ELISA additivity, and neutralization assays. At least two distinct serotype 1-specific conformation-dependent overlapping neutralizing antigenic domains were shown to be present on IBDV-VP2, and were respectively probed by Mabs 3 and 4, and by Mabs 6 and 7. Ten serotype 1 vaccine or pathogenic IBDV strains were tested for neutralization. Most mild or intermediate vaccine strains were efficiently neutralized by all Mabs, whereas US variant A, European pathogenic strain Faragher 52/70 and French hypervirulent isolate 89163 were not neutralized by Mabs 3 and 4. In addition, these two Mabs were shown to bind to the Faragher 52/70 strain, but not to the 89163 isolate, in an antigen-capture ELISA. These results suggest that a neutralizing epitope is possibly modified in European pathogenic IBDV strains, and that Mabs 3 and 4 may prove useful for antigenic differentiation between European classical and hypervirulent isolates.