Attempts to control Aujeszky's disease by vaccination with a glycoprotein-I negative subunit vaccine have been made on nine New Zealand pig farms. Thirty-one to 42 months after the programme of vaccination began, its progress was assessed by measuring the gI-antibody response in pigs from seven of the farms. Three farms had totally eradicated the 'wild' virus infection, one farm was close to achieving complete eradication and the other three farms had made little or no progress. One of the farms which eradicated the 'wild' virus infection achieved this status in two years by combining vaccination with an intensive testing and culling programme; the other two farms had eradicated the 'wild' virus infection by a combination of vaccination and good standards of hygiene without undertaking an intensive culling programme. The farms that had made little or no progress had less satisfactory standards of hygiene and did not practise an intensive testing and culling programme.