The lipopolysaccharide of Haemophilus influenzae is presumed to have a toxic effect on the tracheal epithelium, and then induce a bronchial obstruction. This activity of LPS was studied in vitro on the ciliary beat using a photo-oscillographic apparatus, and in vivo on the rabbit trachea. Neither modification of ciliary beat frequency, nor epithelial damage in the rabbit trachea was observed after a single administration of LPS. In contrast, histopathologic changes were observed in vivo when the intratracheal administration of H. influenzae LPS was followed 24 h later by an intravenous injection of the same LPS. These experimental models seem thus to implicate a Shwartzman type cellular necrosis in the trachea in vivo in the absence of a direct toxic effect of endotoxin itself on trachea in vivo or in vitro.