A field trial to assess the ability of two vaccines to protect calves against respiratory disease was carried out on a large beef rearing unit in southern England over the two winters of 1983 to 1984 and 1984 to 1985. A quadrivalent vaccine containing the killed antigens of respiratory syncytial virus, parainfluenza virus type 3, Mycoplasma bovis and M dispar or a vaccine containing only the respiratory syncytial virus component were inoculated into 246 and 245 calves, respectively; 245 calves remained as unvaccinated controls. The calves were reared in seven batches and outbreaks of disease occurred in five; significant protection was achieved in the four batches in which disease was associated with respiratory syncytial virus and M bovis infection, together or independently. The death rate from pneumonia was 9 per cent in the control group, 2 per cent in the calves inoculated with the quadrivalent vaccine (P less than 0.001), a protection rate of 77 per cent, and 3 per cent in the calves inoculated with the respiratory syncytial virus vaccine (P less than 0.01), a protection rate of 68 per cent. The proportion of calves receiving treatment for respiratory disease was 38 per cent in the control group, 25 per cent in the calves inoculated with the quadrivalent vaccine (P less than 0.001) and 27 per cent in the calves inoculated with the respiratory syncytial virus vaccine (P less than 0.01). The results show that protection against respiratory disease can be achieved by parenteral vaccination of calves with the appropriate inactivated microorganisms.