England Tracheal organ cultures from the human fetus, the chick embryo, and several rodents were used in a study of the effect of Haemophilus infiuenzae on respiratory ciliated epithelial cells. H. infiuenzae grew in medium containing tracheal organ cultures from different sources, resulting in ciliostasis after various periods of time. Supernatant fluids of cultures of H. infiuenzae contained a ciliostatic substance that was active at high dilution in rat tracheal organ cultures. Cultures of human fetus and chick trachea were less sensitive, and some human fetal tracheas were insensitive. Histologic studies showed loss of cilia, damage to epithelial cells, and sloughing. The ciliostatic substance was produced early in the growth cycle of H. infiuenzae, was released into the medium, was nondialyzable, and was stable in heat. This substance was produced by all types and strains of H. infiuenzae tested. The possibility that the ciliostatic substance was an endotoxin was proposed, and the role of this substance in the pathogenesis of infections of the respiratory tract due to H. infiuenzae was considered.