The equine herpesvirus 4 (EHV-4) gene glycoprotein H (gH) gene homologue was localized by virtue of the conserved genomic position of this gene throughout members of the herpesvirus family. The gene maps immediately downstream of the thymidine kinase gene at approximately 0.49 to 0.51 map units within genomic fragment BamH1 C. The EHV-4 gH primary translation product is predicted to be a polypeptide of Mr 94,100, 855 amino acids long, which possesses features characteristic of a membrane glycoprotein, namely an N-terminal signal sequence, a large hydrophilic domain containing 11 putative N-linked glycosylation sites, a C-terminal transmembrane domain, and a charged cytoplasmic tail. Comparison to other herpesvirus glycoproteins revealed identities of 85%, 26% and 32% with the gH counterparts of the alphaherpesviruses EHV-1, herpes simplex virus 1 and varicella-zoster virus, respectively, and of 17% and 18% with those of human cytomegalovirus, herpesvirus saimiri and Epstein-Barr virus. The EHV-4 gH exhibits features previously reported to be conserved throughout the gH polypeptides of herpesviruses of all three subgroups. A region of direct repeat elements and a possible origin of DNA replication are located immediately downstream of the gH gene.