The efficacy of two inactivated and three attenuated Aujeszky's disease virus (ADV) vaccines was compared in pigs in a standardised test. After vaccination, excretion of attenuated vaccine viruses was not detected. All vaccines were capable of preventing mortality, but none conferred complete protection against challenge with a virulent strain of ADV 3 months after the first vaccination. After a single vaccination, no large differences in vaccine efficacy were observed. Revaccination markedly enhanced protection; however, with one vaccine this was not the case. The two inactivated vaccines induced quite different levels of neutralizing antibodies.