The immunogenicity of recombinant (r-) pertactin was examined. Parenteral immunization of mice with natural or r-pertactin produced a similar increase in serum anti-pertactin antibodies and a decrease in Bordetella pertussis lung counts following aerosol challenge. Study of the kinetics of B. pertussis growth in the respiratory tract of immunized and control mice revealed that immunization with r-pertactin halted the multiplication of B. pertussis in the lungs and facilitated the early onset of bacterial clearance. In the trachea, bacterial numbers declined sharply in immunized animals during the first 3 days after challenge but thereafter B. pertussis numbers remained fairly constant throughout the rest of the experiment. Very low doses (0.1 micrograms) of r-pertactin were immunogenic and protective but only if the antigen was absorbed to alhydrogel. In vitro proliferation assays with lymphocytes from mice primed with either natural or r-pertactin indicated that the major T-cell epitopes of pertactin are conserved in the recombinant protein.