Nonopsonized Bordetella pertussis can bind to and become ingested by human monocytes. Previous studies demonstrated that mutant B. pertussis strains that lack fimbriae express a reduced adherence to monocytes. The present study was undertaken to investigate the involvement of the minor fimbrial subunit FimD in the adherence of B. pertussis to human monocytes using purified fimbriae, FimD, and strains with mutations in fimbrial genes. Flow cytometry demonstrated that purified B. pertussis fimbriae avidly bind to monocytes in a dose-dependent manner but bind less to neutrophils and hardly to lymphocytes; FimD-lacking fimbriae did not bind to monocytes. Purified fimbriae or FimD inhibited the binding of wild-type B. pertussis to monocytes in a dose-dependent manner and similarly inhibited the binding of a mutant defective in the major fimbrial subunits. It did not affect the binding of strains defective in FimD. These results prove that B. pertussis bind to human monocytes via the fimbrial minor subunit FimD.