Adhesion of P. aeruginosa to normal and injured rat tracheas was examined. Rat tracheas were injured by exposure to 0.1N HCl for 10 min, and incubated with P. aeruginosa. Adhesion was quantitated by direct count of the number of bacteria attached to a fixed surface area as viewed by scanning electron microscopy. P. aeruginosa adhered to injured tracheas much more than to normal tracheas. The adhesion of P. aeruginosa, preincubated with mucin and sugars, to acid injured trachea was examined. Mucin, N-acetylneuraminic acid and N-acetyl-D-galactosamine inhibited the adhesion of P. aeruginosa to injured tracheas, but not N-acetylglucosamine, L-fucose, D-mannose and D-galactose. Periodate oxidation and neuraminidase treatment of acid injured tracheas reduced the adhesion of P. aeruginosa. These data suggest that N-acetylneuraminic acid (sialic acid) is the receptor for P. aeruginosa or a part of the receptor in acid injured rat trachea and in tracheobronchial mucin.