Vertical transmission of Aujeszky's Disease virus (ADV) was studied in pigs with two British strains. The strains used were both isolated from an outbreak of disease in Yorkshire. In vivo experiments involving intranasal infection of 10-week old pigs revealed that one strain was highly virulent (Leeds-1) while the other was of low virulence (Leeds-2). Both experimental and natural infections were evaluated by the clinical response and by the detection of specific immune responses. The techniques used included class-specific ELISA tests, lymphocyte transformation and antibody-dependent cellular cytotoxicity. Twelve pregnant sows were infected intranasally at 85+/-1 days of gestation. Evidence of transplacental infection was found in only one of the ten surviving sows. The litter consisted of 8 mummified fetuses. ADV antigens were detected in liver sections with fluorescent antibodies. In contrast the incidence of perinatal infection was high. Colostrum-deprived piglets born to infected sows developed antibodies to the virus, providing evidence that they were infected at birth. All naturally-reared piglets, were sero-positive after ingestion of colostrum from their dams. Litters born to sows infected with the low virulence strain were poorly protected and succumbed to the infection, whereas no postnatal mortality was recorded among the litters born to sows infected with the high virulence strain, which induced high levels of colostral immunity. This emphasised the importance of maternal protection in the natural disease and brings into consideration the possibility that such low-virulence strains, which do not evoke clinical disease in mature animals, may be important as the cause of outbreaks of disease in breeding herds. In vitro studies of strain characteristics revealed that sensitivity to trypsin or to heat did not correlate with the virulence of the strains. Studies of the replication of the viruses in the pig alveolar macrophages, however, showed between-strain differences which correlated with their virulence for pigs. The Leeds-1 and Leeds-2 strains were also clearly differentiated by restriction enzyme analysis of their DNA.