Dogs inoculated intranasally with a live avirulent Bordetella bronchiseptica vaccine were monitored for the development of resistance to experimentally induced infectious tracheobronchitis (canine cough). Dogs were challenge exposed with a virulent strains of B bronchiseptica at various times after they were vaccinated. Clinical protection was detectable as early as 48 hours. At postvaccination days 4, 5, and 14, 56%, 83%, and 95% protection was observed. Humoral immunoglobulin (Ig) titers ranged from 1:8.6 on day 0 to 1:147 on postvaccination day 21. In the monitoring of B bronchiseptica-specific secretory IgA by indirect immunofluorescence, titers appeared as early as day 4 after vaccination. The IgA titers ranged from 1:16 on day 4 to 1: 1,024 on day 21. The appearance of IgA titers correlated with the development of resistance to clinical infection.