During its life cycle, surfactant converts from highly surface active, large aggregates to less surface active, smaller aggregates. This process is probably regulated by a serine protease. We tested whether adding alpha1-antitrypsin (alpha1-AT), an antiprotease, to surfactant improves its in vivo function. alpha1-AT was added to Survanta, to a standard phospholipid (PL) mixture, and to a synthetic surfactant (BC mixture = PL mixture + synthetic surfactant proteins B and C) at a dose of 100 mg alpha1-AT per 75 mg PL. Adding alpha1-AT did not affect in vitro surface activity, except for that of the PL mixture. Adult rats were ventilated with 100% O2, at a tidal volume of 7.5 ml/kg and a ventilatory rate of 60 breaths/ min. The rats' lungs were lavaged with saline until the PaO2 dropped below 100 mm Hg, at which time 100 mg/kg of surfactant with or without alpha1-AT or alpha1-AT alone was instilled. After 1 h of ventilation the rats were killed, pressure-volume curves were generated, and the rats' lungs were relavaged. Surfactant treatment improved oxygenation in the order: BC mixture > Survanta > PL mixture. Addition of alpha1-AT equalized oxygenation in all three alpha1-AT groups, but decreased respiratory system compliance in the groups given Survanta and PL mixture. Particle sizing of the final lung lavages showed preservation of large surfactant aggregates after treatment with alpha1-AT. These data suggest that the addition of alpha1-AT to surfactant can exert a positive effect on oxygenation and surfactant metabolism in surfactant-deficient rats.