Agglutinogen 2 (AGG2) of Bordetella pertussis is a fimbrial antigen and therefore a potential adhesin and acellular vaccine component. AGG2 was found to dissociate only under harsh conditions into the subunits of mol. wt. 22500 seen in SDS-PAGE. Results from studies of agglutinogen 3 (AGG3) are presented which confirm previous findings from this Laboratory that AGG3 is also a fimbrial protein but with a subunit mol. wt. of 22000. The amino acid sequence of AGG2, deduced from the nucleotide sequence of the gene encoding it, was used as a basis for synthesis of three peptides. Coupled to Keyhole Limpet Haemocyanin (KLH), the peptides were immunogenic in mice, inducing antibodies which bound well to homologous peptide in ELISA but poorly to intact fimbriae. Monoclonal and polyclonal serotype-specific antibodies failed to react significantly with the peptides or their KLH-conjugates. These results indicate that the synthetic peptides do not represent the serotype 2 epitope. Mice immunized with purified AGG2 or AGG3 were found to be protected against respiratory infection with B. pertussis. Results presented here indicate that this protection is, to a large extent, serotype-specific and that immunization of mice with AGG2 or AGG3 can lead to a change in serotype of the infecting strain. These results are analogous to findings from epidemiological studies of the protection induced in children by whole cell vaccines. They reaffirm the importance of both AGG2 and AGG3 as components of whole cell and acellular vaccines.