The specificity of the cell-mediated immune response to Bordetella pertussis following immunization of C57B1 mice with a whole-cell pertussis vaccine was assessed in a proliferation assay. A proliferative response of lymph node lymphocytes to the filamentous haemagglutinin, the 69 kDa outer membrane protein and the agglutinogens 2 and 3 was demonstrated. The proliferative cells were T cells of the CD4+ phenotype. In addition, several as yet uncharacterized antigens expressed by B. pertussis were shown to induce a proliferative response, also mediated by T cells of the CD4+ phenotype. Although a range of different immunization schedules and preparations of pertussis toxin were used, no specific proliferative responses to pertussis toxin, which is regarded as a protective antigen of major importance from B. pertussis, were found.