Four eight-week-old cats, shown to be free from feline calicivirus, feline herpesvirus and Chlamydia psittaci were challenged with an aerosol of Bordetella bronchiseptica. Within five days the cats developed signs of respiratory disease, characterised by nasal discharge, sneezing, spontaneous or induced coughing and dry or wet rales at auscultation. These signs were present for about 10 days, after which they began to resolve. To test the protective capacity of an experimental fimbrial antigen-based subunit vaccine, 10 kittens were vaccinated twice, with two weeks between the vaccinations, and five kittens were left unvaccinated. Two weeks after the booster the 15 kittens were challenged with an aerosol of B bronchiseptica as the sole pathogen. On the day of challenge the vaccinated kittens had a mean bordetella antibody titre of 2(9.5) whereas the control cats remained seronegative (titre < 2(2)). The control cats developed signs of respiratory disease after challenge, whereas the vaccinated cats were almost completely protected. The degrees of protection against rhinitis, sneezing, spontaneous or induced coughing, and dry or wet rales at auscultation were 100 per cent, 95 per cent, 95 per cent and 100 per cent, respectively. Furthermore, the vaccinated kittens cleared the challenge bacteria more quickly than the controls, resulting in a reduction of 80 per cent on days 15 and 18 after challenge and a reduction of 99 per cent on days 22 and 29 after challenge. The results show that B bronchiseptica can act as a primary pathogen in cats and that a vaccine containing the fimbrial antigen induces a protective immune response.