Immunoblotting was used to identify those Aujeszky's disease virus proteins which elicited major antibody responses in naturally and experimentally infected pigs. Although some proteins comprising purified virus preparations reacted nonspecifically, proteins with mol. wts. of 120 K, 90 K, 71 K and 60 K were antigenically important. These corresponded in size to the virus glycoproteins identified by 3H-glucosamine labelling. Glycoproteins isolated by affinity chromatography from infected PK 15 cells solubilised by Triton X-100 (Triton-soluble glycoproteins) contained antigenic components similar in size to virus glycoproteins and produced minimal non-specific immunoblotting reactions. Antibody responses to the 120 K and 71 K proteins usually occurred together and were more pronounced than responses to other proteins especially at early times postinfection.