The function of pulmonary surfactant of a group of 14 preterm neonates (birth weight 907 +/- 60 g) who suffered from severe respiratory distress syndrome (RDS) and who had received exogenous bovine lipid extracted surfactant on the first day of life was compared to that in a second group of 8 neonates (birth weight 940 +/- 110 g) with mild RDS who had not received surfactant treatment. Mechanical respiratory support from day 2 on was the same in both groups. The minimal surface tension (gamma(min)) improved steadily, falling from about 30 mN/m initially to less than 20 mN/m before extubation, A consistent but loose correlation was found between gamma(min) and mechanical respiratory support necessary, as quantitated by the oxygenation index. Total protein was about 0.8 +/- 0.2 mg/mg of phospholipids and did not change during the first week of life. There were no correlations between total protein and gamma(min) or the oxygenation index. The data suggest that inhibition of surfactant function by proteins leaked into the airspaces does not play a major role during recovery from RDS, Instead, endogenous remodelling of surfactant might be of greater relevance.