This study tested whether adducts formed by covalent linkage of superoxide dismutase (SOD) or catalase to polyethylene glycol (PEG) could augment SOD and catalase activity in alveolar type II cells and document enhanced resistance to oxidant damage. Alveolar type II cells were isolated from adult, pathogen-free rats. Antioxidant enzymes were added to the medium of cell cultures in various concentrations for periods up to 48 h. Incubation with 500 to 3,000 U of PEG-SOD or 10,000 to 40,000 U of PEG-catalase/10(6) cells produced a dose-response-related increase in intracellular enzyme activity in comparison with controls (untreated or treated with SOD or catalase, inactivated PEG-SOD or PEG-catalase, or PEG alone). Uptake was maximal during the first 4 h. Using fluorescent label (fluorescein isothiocyanate) bound to PEG-catalase, we found intracellular localization of the labeled enzyme. Exposure to H2O2 led to reduced cytotoxicity in cells pretreated with PEG-catalase than in controls. We conclude that supplementation with PEG-SOD or PEG-catalase enhanced the activity of these enzymes in alveolar type II cells and increased their resistance to oxidant stress.