The adsorption of pulmonary surfactant to an air/fluid interface is influenced by calcium-dependent interactions between its lipid and protein components. The latter include a glycoprotein of 28-36 kDa (SP-A) and two smaller hydrophobic proteins of 5-8 kDa (SP-B, SP-C). Neutrophil elastase and other proteolytic enzymes found in the alveolar washings in a variety of acute lung injuries may cleave the protein components of lung surfactant. To examine the hypothesis that free airspace elastolytic activity may thereby impair surfactant function, we analyzed the effect of neutrophil elastase on surfactant activity in vitro. The adsorption characteristics of dog surfactant and of complexes reassembled from purified surfactant components were examined after incubations with active or heat-inactivated neutrophil elastase. Surfactant preincubated with the active enzyme showed a marked concentration-dependent slowing of adsorption associated with proteolytic cleavage of SP-A. To determine whether elastase also decreases surface activity by affecting the hydrophobic proteins SP-B and SP-C, we studied the effect of incubating elastase with liposomes prepared from surfactant lipid fractions which contain SP-B and SP-C. The addition of intact SP-A to these liposomes incubated with inactive enzyme immediately enhanced adsorption speed. This enhancement was greatly attenuated in liposomes treated with active elastase, suggesting that one or both of the hydrophobic surfactant proteins had been affected by elastase. We conclude that proteolytic cleavage of surfactant proteins reduces adsorption speed in vitro and may disturb surfactant function in vivo.