Proteins that decrease the surface activity of surfactant accumulate in epithelial lining fluid in respiratory failure. The aim of this study was to isolate a surfactant inhibitor from the airways of rabbits in acute respiratory failure induced by bronchoalveolar lavage (BAL). This inhibitor was identified as being transferrin (TF). Unlike serum TF, TF recovered in respiratory failure was saturated with iron (Fe(3+)-TF). Fe(3+)-TF decreased the surface activity of normal surfactant in vitro, whereas iron-free TF had no effect. In the presence of H2O2 and a reducing agent, Fe(+3)-TF inactivated the surfactant complex: the surface absorption rate was decreased, immunoreactive surfactant protein A was decreased, and malondialdehyde was formed. The acute effects of Fe(3+)-TF and iron-free TF applied to the airways were studied in animal models. In respiratory failure induced by BAL, Fe(3+)-TF deteriorated respiratory failure, whereas iron-free TF had no effect. In respiratory failure induced by hyperoxia for 48 h, administration of iron-free TF ameliorated the respiratory failure and improved the surface activity in BAL. We propose that Fe(3+)-TF accumulating in epithelial lining fluid during lung damage contributes to surfactant inhibition and promotes the formation of free radicals that inactivate the surfactant system.