In the Marek's disease virus (MDV) serotype 2 (MDV2) genome, a gene equivalent to the glycoprotein I (gI) of other alphaherpesviruses was identified and sequenced. The primary translation product comprises 355 amino acids with a M(r) of 38.4 kDa. The predicted amino acid sequence possesses several characteristics typical of membrane glycoproteins, including a N-terminal hydrophobic signal sequence, C-terminal transmembrane and cytoplasmic domains, and extra-cellular region containing three potential N-linked glycosylation sites. Compared to other MDV serotypes, MDV2 gI showed 49% identity with MDV1 gI, and 36% identity with HVT gI at the amino acid level. In transcriptional analyses, a 3.5 kb mRNA which starts between 56 and 147 bps upstream of the potential translational initiation codon of gI was identified as the gI-specific transcript. By a recombinant baculovirus, this potential gI encoding region was expressed as two specific products 45 and 43 kDa. Both products were susceptible to tunicamycin treatment, indicating that they were glycoprotein. Further, the expressed gI reacted with all chicken-antisera raised to each of the three serotypes of MDV (strains GA, SB-1, and FC126), suggesting that gI is expressed by all three serotypes of MDV in infected cells and conserves common antigenic epitope(s) beyond serotypes.