The most highly conserved glycoproteins in herpesviruses, homologues of glycoprotein B (gB) of herpes simplex virus, have been shown to play essential roles in membrane fusion during penetration and direct cell-to-cell spread of herpes virions. In studies aimed at assessing whether sequence conservation is reflected in the conservation of functional properties, we previously showed that bovine herpesvirus 1 (BHV-1) gB was able to functionally complement a gB- PrV mutant. To analyse in detail the function of gB in BHV-1, and to be able to test for reciprocal complementation between pseudorabies virus (PrV) and BHV-1 gB, we isolated a gB- BHV-1 mutant on a cell line stably expressing BHV-1 gB. Functional analysis showed that BHV-1 gB was essential for penetration as well as for direct cell-to-cell spread of BHV-1, indicating similar functions for PrV and BHV-1 gB. However, PrV gB was unable to complement plaque formation, i.e. direct cell-to-cell spread, or penetration of gB-BHV-1 virions despite its incorporation into the virion envelope. Analysis of cell lines expressing chimeric gB molecules composed of PrV and BHV-1 gB showed that plaque formation of both gB- mutants was complemented when the carboxy-terminal half of the chimeric gB was derived from BHV-1 gB and the amino-terminal half from PrV gB. In the opposite case, unidirectional complementation occurred. Although the chimeric molecules were generally less efficient in complementing infectivity of free virions, a similar complementation pattern was observed. In summary, our data show a unidirectional pattern of transcomplementation between the gB glycoproteins of PrV and BHV-1. This indicates that these proteins are functionally related but not identical. The unidirectional transcomplementation pattern was determined by the provenance of the carboxy-terminal half in chimeric gB proteins indicating that regions which are important for gB function but differ between PrV and BHV-1 reside in this part of gB.