Immunogold electron microscopy and monoclonal antibodies (Mabs) were used to localize surface-related antigens of Bordetella pertussis. Unfixed organisms of B. pertussis strains which are included in the Danish whole-cell pertussis vaccine and fixed cells from a vial of vaccine were examined. Mabs to agglutinogens 2 and 3 labelled fimbria-like structures on both live and fixed cells in a serotype-specific manner. Mab against pertactin, a 69 kDa outer membrane protein, produced intense labelling of the surface of unfixed cells, whereas staining was reduced when fixed cells were examined. Mabs against filamentous haemagglutinin (FHA) stained aggregates of material between or adherent to both live and fixed cells. Negligible labelling of FHA on cell surfaces was observed. Mabs to pertussis toxin and adenylate cyclase toxin labelled loose-structured material which was adherent to or between cells, but neither of these toxin antigens was expressed on the surface of B. pertussis in Mab recognizable form. It is therefore suggested that these antigens are readily dispersed after exit from the outer membrane of B. pertussis.