Infection with bovine herpesvirus type 1 (BHV-1) occurs worldwide and causes serious economic losses due to the deaths of animals, abortions, decreased milk production, and loss of body weight. BHV-1 is frequently found in bovine semen and is transmitted through natural service and artificial insemination. The detection of BHV-1 in bovine semen is a long-standing problem in veterinary virology which is important in disease control schemes. In the present study, ordered deletions of the full-length BHV-1 glycoprotein open reading frame were used to identify an epitope recognized by a specific monoclonal antibody (MAb). A glycoprotein D fragment containing this epitope was then amplified using an in vitro protein amplification assay developed previously (J. Zhou, J. Lyaku, R. A. Fredrickson, and F. S. Kibenge, J. Virol. Methods 79:181-189, 1999), and the resulting peptide was detected by indirect enzyme-linked immunosorbent assay (ELISA) with the specific MAb. This method detected 0.0395 50% tissue culture infective dose of BHV-1 in raw bovine semen, which was 1,000-fold more sensitive than traditional PCR. We therefore conclude that this in vitro protein amplification assay combined with ELISA has superior sensitivity for direct virus detection in clinical samples.Bovine herpesvirus type 1 (BHV-1) is responsible for a variety of diseases in cattle, including respiratory and genital infections, conjunctivitis, abortion, and enteritis, causing great economic loss to the cattle industry worldwide (6). As in other alphaherpesviruses, BHV-1 glycoproteins are the major structural components of the viral envelope and virus-infected cell membranes. The glycoproteins play important roles in viruscell interactions, including recognition and attachment of the virion and its penetration into susceptible cells (8,10,12), viral neutralization, and immune destruction of infected cells (7,13). Glycoprotein D (gD) of BHV-1, a homologue of herpes simplex virus gD, is one of the four essential major glycoproteins, together with gB, gC, and gH, which have been identified on the virus envelope and the plasma membrane of BHV-1 infected cells (22). It stimulates a potent neutralizing antibody response in animals and induces significant protection against BHV-1-induced diseases. Moreover, BHV-1 gD is a very stable antigen whose epitopes do not change under selective pressure (18).It has also been reported that monoclonal antibodies (MAbs) against gD show the highest complement-independent virus-neutralizing activity and inhibit virus adsorption and penetration (4, 9, 23). MAbs against BHV-1 gD and their use in diagnostic tests, epitope mapping, and functional analysis have been reported by several investigators, including van Drunen et al. (22) BHV-1 is frequently found in bovine semen and can be widely transmitted through artificial insemination. The detection of BHV-1 in bovine semen is a long-standing problem in veterinary virology which is important in disease control schemes. None of the methods developed so far have been found who...