Comparative Studies on Virulence of Aujeszky's Disease Virus Strains Clinical, virological, pathomorphological and immunofluorescence examinations were performed in 24 piglets, 4 of them being experimentally infected with a virulent (B) and 6 of them with a low virulent virus strain of Aujeszky's disease (K). 14 animals were kept in enzootically infected practice stock for contact exposure. Results revealed differences in clinical and morphological signs of the disease in several experimental groups due to different virulence and tissue tropism of the used virus strains. Piglets infected with strain B showed clinical changes characteristic for the disease. In animals infected with strain K no clinical signs could be observed in the same observation period. In contact animals only respiratory disorders occurred. Histological changes of the central nervous system were established in animals of all experimental groups. In animals infected with strain K and in the contact animals in addition there were ascertained morphological changes of the interstice of the lungs in form of interstitial pneumonia of the lymphohistiocytic types. Low virulent strains are proved to show a distinct pneumotropism.