Six collie dogs, eight weeks old, were inoculated intramuscularly with an aluminium hydroxide adjuvanted preparation of killed Bordetella bronchiseptica; the inoculation was repeated after two weeks. Two weeks after the second inoculation, the vaccinated dogs and a comparable group of six unvaccinated animals were challenged by exposure to an aerosol of pathogenic B bronchiseptica. All six unvaccinated control dogs developed respiratory disease characterised by persistent coughing. In contrast, four of the vaccinated dogs remained free from clinical respiratory disease while, in the other two dogs, disease was less severe and of shorter duration than in controls. At necropsy, there were only slight changes in the lungs of vaccinated dogs but in controls there was a severe tracheobronchitis with areas of exudative pneumonia. Bacteriological examination showed a marked reduction in the numbers of B bronchiseptica isolated from the respiratory tract of vaccinated animals compared with controls. An aluminium hydroxide adjuvant vaccine may be of value in controlling naturally occurring respiratory disease in dogs in which B bronchiseptica is involved.