A gene in equine herpesvirus 1 (EHV-1; equine abortion virus) equivalent to the gB glycoprotein gene of herpes simplex virus (HSV) has been identified by DNA hybridization and nucleotide sequencing. A 4.3 kbp EHV-1 PstI-ClaI sequence (0.40 to 0.43 map units) contained an open reading frame flanked by appropriate control elements and was capable of encoding a polypeptide of 980 amino acids. This had 50 to 60% identity over a 617 amino acid conserved region with the gB gene products of HSV and three other alphaherpesviruses, and 20 to 30% identity with those of human cytomegalovirus and Epstein-Barr virus. Analysis of the amino acid sequence predicts a long signal peptide, hydrophobic and hydrophilic domains and N-glycosylation sites, and has identified a probable internal proteolytic cleavage site. The EHV-1 gB open reading frame appears to be overlapped at its 5' end by 135 nucleotides of the 3' end of an upstream open reading frame the potential translation product of which has approximately 50% identity with HSV gene ICP 18.5 and VZV gene 30 products.