Strains of Bordetella pertussis, Bord. parapertussis and Bord. bronchiseptica adhered readily to live cultures of human fibroblasts. Sera from animals immunised against Bord. pertussis contain antibodies that specifically prevent the adhesion of Bord. pertussis to the cells. Anti-adhesion antibodies will not detach Bord. pertussis already adherent to fibroblasts. Limited experiments suggest that the same conclusions are true for Bord. pertussis and monkey tracheal epithelium. It is concluded that the adhesion of Bord. pertussis to cells depends on a biochemical mechanism, and is analogous to the adsorption of influenza virus on red cells.