[35S]methionine-labeled Bordetella pertussis adhered to monolayers of WiDr cells, an epitheliumlike cell line from a human intestinal carcinoma. Adherence was proportional to the density of the WiDr cells and to the concentration of B. pertussis in the assay. Adherence of virulent phase I strains Tohama phase I, 114, and BP338 was much greater than adherence of avirulent strains Tohama phase III and 423 phase IV. Mutants deficient in the production of the filamentous hemagglutinin (FHA) were hemagglutination negative and adhered to WiDr cells much less efficiently than the parent strains. Preincubation of B. pertussis cells with FHA increased their hemagglutination activity and adherence to WiDr cells. Goat antibody to FHA inhibited, in a dose-dependent manner, the adherence of strain Tohama I but not the adherence of FHA-deficient mutant Tohama 325. At similar protein concentrations, normal goat antibody, goat antibody to pertussis toxin, or the Fab fragments of goat antibody to serotype 2 fimbriae had no effect on adherence. Also, an FHA-positive strain without fimbriae showed high adherence, while a fimbriated FHA-deficient mutant adhered poorly. Our data indicate that FHA plays a major role in adherence of B. pertussis to human WiDr cells. Fimbriae do not appear to mediate attachment of B. pertussis to WiDr cells.