Immunoelectron microscopy using colloidal gold-tagged antibodies was used to detect filamentous hemagglutinin (FHA) and pertussis toxin (PT) antigens on the surface and in the cytoplasm of Bordetella pertussis cells. Both gold-tagged antibodies to FHA and PT labeled the aggregates of filamentous material on the surface of sediment-settled phase I cells under static conditions. FHA and PT antigens were detected also on ultrathin sections made after embedding the phase I cells in Lowicryl K4M resin. On the ultrathin sections, intense label of gold-tagged antibodies to FHA and PT was present on the cell surface and also in the cytoplasm, but not in the nucleoid. The aggregates of filamentous material adhering on the surface of phase I cells were most abundant on culture day 3, the end of the logarithmic growth stage, but most of the aggregates were found detached from the cell surface on culture day 5 or 7, the stationary stage. The aggregates were not found on the surface of phase III cells. The present study suggested that FHA and PT antigens were localized on the same cell structure and that both antigens were synthesized in the cytoplasm and secreted across the cell membrane mainly in the logarithmic growth stage of the phase I cells.