Bordetella pertussis synthesizes several factors. It has been suggested that one of these factors, the adenylate cyclase-hemolysin (AC-Hly), directly penetrates target cells and impairs their normal functions by elevating intracellular cAMP. In the present study, we show that active immunization with purified B. pertussis AC-Hly or AC (a fragment of the AC-Hly molecule carrying only the adenylate cyclase activity but no toxin activity in vitro) protects mice against B. pertussis intranasal infection. Immunization with AC-Hly or AC significantly shortens the period of bacterial colonization of the mouse respiratory tract. Furthermore, B. parapertussis AC-Hly or AC are also protective antigens against B. parapertussis colonization; their protective activities are equivalent to that of the whole-cell vaccine. These results suggest that AC-Hly may play an important role in Bordetella pathogenesis, in a murine model. If this factor plays a similar role in the human disease, its use as a protective antigen could reduce not only the incidence of the disease, but also the asymptomatic human reservoir by limiting bacterial carriage.