A study was undertaken of the protection induced by inactivated and live Aujeszky's disease virus vaccines. The vaccines were administered using a special device which, without the use of a needle, delivered the preparation intradermally. The trials were performed on 88 pigs which were vaccinated at the beginning of the fattening period both in experimental conditions and in pig herds. All the pigs were challenged at the end of the fattening period in isolation units. The results obtained were compared with those obtained using the same vaccines injected intramuscularly. It was shown that vaccination via the intradermal route induced good protection in the vaccinated animals and was similar to that conferred by live virus vaccine injected intramuscularly. The results, with the inactivated virus vaccine, were not so good when it was injected via the intradermal route. Studies with intradermal vaccination showed no local lesion or very small nodules strictly localized to the dermis. The results also confirmed that the effects of challenge exposure depended on the health status of animals prior to infection and show the necessity to use a synthetic value (delta G) to interpret the data and mainly to compare the results objectively. In fattening pigs this vaccination procedure is attractive because (i) less animal constraint is needed than would be for intramuscular injections, (ii) injection can be checked by the presence of a visible papula at the site of inoculation and, (iii) pigs can be vaccinated in the ham while they are feeding. Injection without a needle also contributes to avoiding bacterial contamination under practical farm conditions of vaccination.