The primary structure of glycoprotein B (gB) is conserved strongly among many members of the Herpesviridae, including some that differ vastly in their natural properties. To determine whether the structural similarity between the gBs of herpes simplex virus type 1 (HSV-1) and bovine herpesvirus type 1 (BHV-1) was reflected in functional homology, we constructed pseudodiploid HSV-1 virions which, in addition to their own gene encoding gB, also contained a gene for encoding BHV-1 gB. Two kinds of pseudodiploid viruses were constructed. In one, the coding sequences of the BHV-1 gB gene were linked to the 5' flanking sequences of the HSV-1 thymidine kinase (TR) gene. In the other, the entire BHV-1 gB gene, including its own flanking sequences, was introduced into the TK gene. In cells infected with the viruses both HSV-1 and BHV-1 gB were made but they could be distinguished immunologically by monoclonal antibodies. Both glycoproteins were inserted into cellular and virion membranes but did not form oligomers with each other. A monoclonal antibody that binds to HSV-1 gB but not BHV-1 gB neutralized the parental HSV-1 and a revertant pseudodiploid virus from which the gene encoding BHV-1 gB had been excised, but was significantly less efficient at neutralizing the pseudodiploid viruses. This suggests that the BHV-1 homologue can complement the HSV-1 gB functions required for infectivity.