Humoral and cellular immune responses were analysed in mice inoculated intranasally with Bordetella bronchiseptica. After infection, the number of bacteria that colonized the respiratory tract of the mice increased during the first day and decreased thereafter. Total IgG levels increased as early as 14 days after infection and decreased with time after infection, whereas total IgA and IgM levels were lower but remained stable. Specific antibodies to the bacteria were mainly IgG2a and IgA and persisted up to 10 months after infection. Some of these specific antibodies were directed against adenylate cyclase-haemolysin, the bacterial factor that had been shown to be necessary for initiation of infection. The proliferation of Bordetella bronchiseptica-reactive spleen cells occurred during the acute phase of infection. T cells from infected mice produced increasing amounts of IFN gamma and IL-2 after infection. Although very low levels of IL-10 were produced, no IL-4 was detected after bacterial stimulation in vitro. These results suggest that Bordetella bronchiseptica infection induces primarily a Th1-type T-cell response. Importantly, the authors demonstrated that antibody and T-cell responses directed against bacterial determinants of the virulent strain and to purified adenylate cyclase-haemolysin were long-lasting. This observation could be due to the fact that Bordetella bronchiseptica may persist intracellularly in the host as it was demonstrated in vitro.