The effect of dexamethasone on infectious bovine rhinotracheitis virus (Bovine herpesvirus 1: BHV 1) was studied by measuring the mean plaque size produced by eight virus strains under two concentrations of dexamethasone phosphate (0.1 and 1 mM). Dexamethasone induced a significant reduction of the mean plaque size, whatever the strain used. Some differences were noted between the BHV 1 strains studied, depending on the dexamethasone concentration.