A blocking ELISA was developed to distinguish between Aujeszky's disease virus (ADV)-infected and vaccinated pigs, on the basis of presence or absence of serum antibodies to glycoprotein I (gI) of ADV. The gI-ELISA detects antibodies that block the reaction of monoclonal antibodies to one or two epitopes on gI of ADV. The ADV-gI antibody response appeared between one and two weeks post-infection and persisted at a high level for at least seven months. Five of the nine ADV-vaccine strains examined were found to be "gI-negative". Pigs vaccinated with a gI-negative vaccine did not develop an ADV-gI antibody response until they were challenge-exposed to a virulent strain of ADV. The gI-ELISA is highly specific, sensitive and suitable for large-scale sero-epidemiological studies to identify infected pigs in populations vaccinated with gI-negative vaccines. The gI-ELISA provides, therefore, a basis for ADV-eradication programmes, which introduces a novel concept in the control of animal virus diseases.