It has recently been shown that the antibody response to glycoprotein I (gI) of Aujeszky's disease virus can be used to distinguish infected from vaccinated pigs. To examine whether pigs exposed to low doses of a mildly virulent strain of Aujeszky's disease virus produce antibody to gI four groups of four pigs were inoculated intranasally with 10, 10(2), 10(3) or 10(4) plaque forming units (PFU) of the Sterksel strain. Two unvaccinated pigs and two pigs vaccinated intranasally with Bartha's K strain, a gI-negative vaccine, were placed in contact with each group. The pigs given 10 PFU and the in-contact pigs in this group did not become infected. The inoculated and the unvaccinated in-contact pigs in the other groups developed mild signs of illness and produced antibody to gI. Four of six vaccinated in-contact pigs that became infected showed neither clinical signs nor virus shedding and still produced antibody to gI. The other two vaccinated pigs appeared to be resistant to contact-challenge. The antibody response to gI persisted for at least seven months. These results support the idea that Aujeszky's disease virus may be eradicated by a programme based on vaccination with gI-negative vaccines, in conjunction with the detection and subsequent removal of gI-antibody positive, infected, pigs.