Aujeszky's disease is primarily a disease of pigs caused by a herpesvirus. In pigs, the primary site of virus replication is the nasopharynx. Some strains cause lesions only in the central nervous system, others cause rhinitis, pneumonia and encephalitis. Colostrum from immune sows protected 3 day old piglets from experimental infection, but did not prevent the piglets excreting virus. Colostral antibodies were 11-16-fold greater than the serum titres in the sows and the half life of passiveantibody was 8.5 days. Pigs given virus vaccine intramuscularly did not excret vaccine virus whereas live vaccine virus given intranasally was excreted. Although only low titres of serum neutralizing antibody were detected, vaccination gave good protection against severe experimental challenge; the intranasally vaccinated group was better protected. A naturally occurring local strain and temperature-sensitive mutants are being examined as potential vaccines.