The nature of the receptors for mucoid and nonmucoid Pseudomonas aeruginosa was investigated by using adherence to injured tracheal epithelium as a model. Bovine submaxillary mucin and crude rat tracheal mucin inhibited the adherence of both types of P. aeruginosa. Among the sugars present in these mucins only N-aceylneuraminic acid inhibited adherence. Inhibition of adherence probably involved the binding of N-acetylneuraminic acid to the bacterial cells and not to the tracheal cells. The mucoid strain appeared to be much more sensitive to inhibition by N-acetylneuraminic acid. Periodate oxidation and cholera filtrate also reduced the adherence of both strains, but Clostridium perfringens neuraminidase treatment did not alter adherence. A nonmucoid isogenic mutant of an unstable mucoid strain was also inhibited by N-acetylneuraminic acid. These data suggest that the receptor for P. aeruginosa is a sialic acid moiety on cell surfaces or in mucins.