Bordetella pertussis, the causative agent of whooping cough, adheres specifically to human ciliated respiratory-epithelial cells. In order to identify the adhesins of Bordetella, we tested mutant strains, each deficient in one of several bacterial virulence factors, for the ability to adhere to single, human ciliated cells in vitro. Loss of adherence was associated with lack of secretion of either of two surface antigens: filamentous hemagglutinin or pertussis toxin. These strains regained adherence if both components were present in the assay system (i.e., the missing protein was supplied exogenously). In this instance, both bacterial components were found bound to the surface of all the Bordetella strains tested as well as to cilia. We hypothesize that filamentous hemagglutinin and pertussis toxin, two naturally secreted surface antigens, act in concert as adhesins of Bordetella for human cilia.