The gene for the homologue of herpesvirus glycoprotein B (gB) has been identified in the genome of human herpesvirus 6 (HHV-6), strain U1102, and the nucleotide sequence was determined. The open reading frame encodes a protein of 830 amino acids (93.2K) with the characteristics of a transmembrane glycoprotein and close similarity to the gp58/116 complex of human cytomegalovirus (HCMV). Monoclonal antibodies 2D10 and 2B9 have been shown previously to react with an HHV-6 glycoprotein of apparent M(r) 112K, and its proteolytic cleavage products of M(r) 64K and 58K. We show that both monoclonal antibodies detect prokaryotically expressed carboxy-terminal fragments of the HHV-6 gB homologue. This indicates that the HHV-6 gB homologue is probably processed by proteolytic cleavage similar to its equivalents in HCMV and various other herpesviruses.