Active immunity against Aujeszky's disease virus (ADV) was compared at the end of the fattening period in pigs which had been vaccinated with the attenuated Bartha strain according to different schedules in the presence of different levels of maternal immunity. The percentage of seropositive pigs at the end of the fattening period varied from 21 to 94 per cent. The percentage was significantly higher when the vaccination schedules were applied to pigs from mothers vaccinated with an attenuated strain compared to pigs from mothers vaccinated with a subunit vaccine or from infected-immune mothers. Additionally, this percentage was two to three times lower when pigs were vaccinated once at 10 weeks old compared to pigs either revaccinated at 14 weeks or vaccinated once at 14 weeks old. When the virus strain used for vaccination had been suspended either in saline or in an oil-in-water emulsion, significant differences were not found in the serological response after vaccination and in the reduction of virus excretion upon subsequent challenge. In challenge experiments, a significantly longer duration of virus excretion was observed in vaccinated pigs which had not seroconverted than in vaccinated but seropositive pigs. The vaccination schedules for sows and fattening pigs in view of the eradication of ADV are discussed.