Kennel cough, also called canine infectious tracheobronchitis, is a common infectious and highly contagious disease of dogs. A number of different infectious agents, often in combination, can be responsible for the disease, however Bordetella bronchiseptica, and canine parainfluenza virus are generally regarded as the two most commonly implicated pathogens. Although rarely fatal, the disease can cause persistent, often severe, bouts of coughing which can last many weeks and cause distress to both dog and owner. Therefore many vaccines have been developed in an attempt to prevent clinical disease and reduce shedding of infection following exposure. Although injectable vaccines are available, and are often preferred by vets because of the ease of administration, intranasal vaccines are felt to offer a number of potential advantages. In this respect, one of the often-stated advantages of intranasal vaccination is that it is less susceptible to interference by circulating maternally-derived antibody (MDA) and therefore can potentially be used to immunize very young puppies. However a statement warning about the possibility of MDA interference can be found on the product literature for at least one intranasal kennel cough vaccine and therefore it may be that this generally accepted dogma is not true - at least in the case of certain products. This study therefore was instigated in order to establish whether protection against Bordetella bronchiseptica in the presence of significant levels of passive antibody could be demonstrated for one particular intranasal vaccine. Thirty three SPF puppies, both seronegative and culture negative for Bordetella bronchiseptica, were used in this study. At two days of age all puppies were injected with Bordetella bronchiseptica-positive antiserum to provide levels of passive circulating antibody which was well above the mean titre for adult dogs in the field. At three weeks of age 17 pups were vaccinated intranasally with Nobivac® KC plus (Intervet) - a trivalent vaccine containing live attenuated B. bronchiseptica strain B-C2, canine parainfluenza virus strain Cornell and canine adenovirus 2 strain MH1 components, diluted to its minimum authorised titre (106.5 CFU/dose) in respect of the Bordetella component. The remaining 16 puppies were kept as unvaccinated controls. At 96 hours after vaccination the puppies were challenged with virulent B. bronchiseptica. After challenge the dogs were evaluated over a period of 25 days for respiratory signs using a numerical clinical scoring system. Just before challenge and at regular times after challenge, nasal swabs were collected for Bordetella isolation. After challenge, both vaccinates and controls developed some upper respiratory tract signs, as expected from previous vaccine challenge studies. However the total clinical scores were significantly lower in the vaccinated group compared to the controls (p=0.0022). Until four days post-challenge the mean body temperature of vaccinates and controls were nearly identical. However, from day 5 post-challenge onwards, the controls had a significantly higher average body temperature than the vaccinates (p=0.0155). On the day of challenge, B. bronchiseptica was isolated from 8 of the17 dogs that were vaccinated with Nobivac KC plus, but not from any of the control dogs. At day three after challenge, the vaccinates still had a higher re-isolation rate compared to the controls - probably due to shedding of the live vaccine strain. However from day 5 to 25 post-challenge, the controls had a significantly higher overall re-isolation rate than the vaccinates (P=0.0467). From the results it can be concluded that Nobivac KC plus at the minimum titre is effective in reducing clinical signs and bacterial shedding in passive antibody positive 3-week-old puppies challenged with BordeteIla bronchiseptica, 96 hours after vaccination.