A type-specific monoclonal antibody was produced by immunizing mice with purified equid herpesvirus-1 (EHV-1). The EHV-1 specific mAb reacted with all the EHV-1 strains tested so far by indirect ELISA, immunofluorescence, and immunoperoxidase tests. No reactions were detected with the EHV-4, EHV-2, or EHV-3 strains tested. The indirect immunofluorescence and immunoperoxidase tests showed that the nuclei of infected cells were predominantly stained by this mAb. Triton treatment of the virus and immunogold labeling experiments indicated that the nucleocapsid of EHV-1 was the target antigen of the mAb. Preliminary results indicated that this mAb might be a useful tool in detecting specific antibody in horses that have been exposed to EHV-1. In a blocking ELISA, antibodies in sera from hamsters, mice and a foal which had been exposed to EHV-1 were differentiated from those in sera of animals exposed to EHV-4.