The value of a skin test for the diagnosis of Aujeszky's disease (pseudorabies) in swine was examined. Semipurified and concentrated antigens, obtained by ether/tween 80 inactivation of Aujeszky's disease virus, were inoculated intradermally into seronegative, experimentally infected and vaccinated swine and into pigs with maternal antibodies. A specific skin reaction, characterized by a visible indurated swelling, was observed within 24 hours in animals with active immunity, No reaction was seen in pigs with maternal immunity or in seronegative uninfected and unvaccinated animals. In the infected and vaccinated groups, 89% and 58% respectively of the animals with seroneutralizing antibodies were positive by the skin test response. Positive reactions were observed as early as 8 days after the infection. All but one seronegative animals remained free of antibodies after 2 consecutive skin tests and the course of decline of maternal antibodies was not changed. An earlier skin test did not lead to sensitization for a later application. The present results, together with field experience on 8 farms, revealed that the skin test could be considered a reliable method for diagnosis of Aujeszky's disease in swine on a herd basis.