The nucleotide sequence of the gene to the left of the gI gene of equine herpesvirus 4 (EHV-4) was determined. The gene encodes a peptide of 402 amino acids with an unprocessed M(r) of 45,323. The predicted polypeptide has several features of a glycoprotein including a hydrophobic signal sequence, a membrane spanning domain and four potential N-linked glycosylation sites within the proposed external domain. The predicted amino acid sequence of EHV-4 gD shows 83% identity with that of equine herpesvirus 1 gD. Conservation of the tertiary structure is suggested by the alignment of six cysteine residues with those of the gD of six other alphaherpesviruses. Screening a lambda gt11/EHV-4 expression library with monoclonal antibodies against several of the most abundant EHV-4 glycoproteins unequivocally identified the protein encoded by the EHV-4 gD gene as gp17/18.