We have previously demonstrated that recombinant human copper-zinc superoxide dismutase (rhCu,ZnSOD) is rapidly incorporated into cells of airways, respiratory bronchioles, and alveoli after intratracheal administration. The present study examines whether this cellular uptake is specific for rhCu,ZnSOD or whether other proteins are similarly incorporated into lung cells. Twenty-two newborn piglets (2-3 days old, 1.2-2.0 kg) were intubated and mechanically ventilated. Eight piglets received fluorescently labeled recombinant human manganese superoxide dismutase (rhMnSOD), six received fluorescently labeled albumin, two received free (unbound) fluorescent label intratracheally, and two piglets served as untreated controls. To determine whether endogenous surfactant was important in the process of intracellular uptake, four additional piglets were made surfactant deficient by repeated bronchoalveolar lavage and then given rhCu,ZnSOD intratracheally. All animals were killed after 30-60 min. Lung sections were examined blindly by laser confocal microscopy. Similar to our previous observations with rhCu,ZnSOD, intracellular uptake of rhMnSOD and albumin was noted throughout the lung. The free label did not localize intracellularly. The uptake of proteins did not appear to be affected by surfactant deficiency. rhMnSOD administration was associated with a greater than twofold increase in lung MnSOD activity. Data suggest that the cellular uptake of antioxidants and other proteins in the lung may reflect a nonspecific host defense system for clearing proteins from the lumen of airways and alveoli.