In previous experiments, we have demonstrated that the presence of equine herpesvirus 2 (EHV-2) enhanced plaque formation in cell cultures infected with equine herpesvirus type 1. To determine whether a specific region of the EHV-2 genome is responsible for this effect, we have constructed a library of Bam HI fragments of the EHV-2 genome ligated into pcDNA plasmid. Equine dermal (ED) cell cultures were subsequently transfected with the constructs, passaged 5 times, tested for the presence of the plasmids and infected with EHV-1 at MOI = 0.01. Only in cultures transfected with the pcDNA/Bam HI[G]construct, designated delta2/4, the mean number of plaques at 24 hrs p.i. was approximately 10 times higher than in non-transfected controls. Virus titers in culture supernatants as well as in freeze-thawed cells were 4- and 5-fold higher, respectively, than in non-transfected cultures. These differences were observed only at 24 hrs p.i. At 48 hrs p.i. cultures were completely destroyed and, surprisingly, the virus titer was slightly lower in the supernatant of transfected cells. However, the titer of EHV-1 in freeze-thawed culture was exactly the same as in the control. These results suggest that the presence of Bam HI[G] fragment of the EHV-2 genome stimulates (accelerates?) plaque formation only at earlier stages of infection but does not influence the total yield of EHV-1 at 48 hrs p.i. The exact mechanism of this stimulation remains unclear and further experiments are necessary to determine the role of putative EHV-2 proteins encoded by Bam HI [G] fragment of the EHV-genome.