The effect of prostaglandins PGE2 and PGF alpha 2 on infectious bovine rhinotracheitis (IBR) virus (bovine herpesvirus 1; BHV-1) was studied by using the measurement of the mean size of plaques produced in cell culture by IBR/Cu5 strain. Three concentrations (0.1, 1 and 10 micrograms/ml) of prostaglandins, PGE2 and PGF alpha 2 were used. Each prostaglandin provoked an increase in the mean plaque size when compared to control plaques, at the concentrations 1 and 10 micrograms/ml for PGE2 and 10 micrograms/ml for PGF alpha 2.