Bronchoalveolar lavage (BAL) fluid was obtained from 24 sequentially studied patients with adult respiratory distress syndrome (ARDS) for assessment of potential activating and mediating factors. Proteolytic activity of the fluids was observed by measuring cleavage of radiolabeled proteins of the contact (Hageman factor) and complement systems. Proteolytic activity was observed in 17 of 24 (71%) patients with ARDS, and BAL fluid of the 7 ARDS patients without demonstrable, active, enzyme exhibited inhibitory activity for the proteolytic activity. The enzymes cleaved Hageman factor, prekallikrein, plasminogen, high molecular weight kininogen, C4, C3, C5, and Factor B of the complement system. Cleavage of the contact system proteins producted fragments similar or identical in size to the fragments observed during activation of these molecules, although continued incubation invariably reduced the protein to small peptide fragments. None of 7 normal individuals, and 29 of 99 patients (29%) with other forms of pulmonary disease contained measurable enzymes. The proteolytic activity in BAL fluid of ARDS patients was blocked by diisopropylphosphofluoridate (0.1 mM), Trasylol, soybean trypsin inhibitor, and normal plasma, or plasma deficient in inhibition of the first component of complement. Alpha(1)-proteinase inhibitor (alpha1-PI)-deficient plasma failed to inhibit the proteolytic activity and addition of alpha1-PI to the deficient plasma reconstituted the inhibition. MUCH OF THE PROTEOLYTIC ACTIVITY OF THE BAL FLUID FROM ARDS PATIENTS WAS IDENTIFIED AS NEUTROPHIL ELASTASE: the fluids cleaved elastin and synthetic peptide substrate of neutrophil elastase, neutrophil elastase antigen was present in the BAL fluids as determined immunologically using antineutrophil elastase, alpha1-PI was the major inhibitor in plasma, and the enzyme was inhibited by diisopropylphosphofluoridate but not chelation. In addition, purified neutrophil elastase produced cleavage fragments of proteins of the contact system similar to those of the BAL fluids. In each of the seven BAL fluids of ARDS patients that did not reveal active elastase, alpha1-PI was present in active form (as determined by (125)I-trypsin binding). In 9 of the 17 patients with active elastase in the BAL fluid, alpha1-PI antigen was present in the fluid, but was inactive (no binding of (125)I-trypsin). Immunoelectrophoretic analysis of elastase and alpha1-PI throughout proteins in these BAL fluids revealed the presence of both elastase and alpha1-PI that migrated with the same R(f), suggesting the presence of an enzyme-inhibitor complex. Free, inactive alpha1-PI was also observed in these fluids. The data reveal that in BAL fluids from all 24 patients with ARDS, leukocytic elastase and/or alpha1-PI exist. A complex of elastase and alpha1-PI was observed in BAL fluids, and in some cases where active enzyme and alpha1-PI coexisted, free, but inactive alpha1-PI was present.