Studies to investigate the efficacy of an inactivated vaccine against infectious bovine rhinotracheitis (IBR) suggest that this vaccine can prevent the in utero infection of calves from experimentally infected dams. In an experimental herd the inactivated vaccine induced a humoral immune response in both seropositive and seronegative cattle and, after subsequent intratracheal infection with IBR (BHV-1) virus, prevented development of symptoms in the cows and protected their fetuses against infection. The calves were all healthy and were born at term. The non-vaccinated, seronegative cows responded to the experimental infection with mild respiratory disease and abortion of 4 out of 10 fetuses. All organs from the aborted fetuses were found to have IBR virus. Through the use of this vaccine, the nucleus of a seronegative, virus-free breeding herd can be established. Thus, valuable genetic material can be preserved and the eradication of IBR becomes a realistic prospect. From our initially strictly controlled experiments producing 234 healthy calves, our programme was expanded into farm practice where 1001 calves were reared free from IBR virus.