To study the development and persistence of circulating antibodies directed against bovine herpesvirus-1 (BHV-1) induced by vaccination, approximately 80 per cent of the seronegative cows in four partly seronegative dairy herds were vaccinated once with a temperature-sensitive live virus vaccine. Most (83 per cent) of the vaccinated animals developed antibodies to BHV-1 within two months after the vaccination. In the same period, 21 per cent of the unvaccinated control cattle also seroconverted, suggesting that the vaccine virus had been transmitted to them. Thirty months after they had been vaccinated 91 per cent of the vaccinated animals which responded still had detectable antibodies. The results suggest that vaccine-induced antibodies may persist for years and thus may interfere with control programmes for BHV-1 which are based on serological monitoring.