Experiments performed in vitro have demonstrated that leukocyte neutral proteases produce an important mediator of inflammation, C5a, by proteolysis of the C5 component of the complement system. Cystic fibrosis (CF) lung fluids were characterized by high levels of neutrophils (39% of total cells versus 2% in normals) and contained significantly elevated amounts of elastolytic activity (mean 17.7 ng/micrograms total protein) compared to the lung fluids obtained from normal volunteers (0.2 ng elastolytic activity/micrograms protein, p = 0.001). The objective of these studies was to determine if complement activation and complement-derived chemotactic activity are present in CF lung fluids. C3c peptide representing activation of C3 could not be identified in the bronchial-alveolar lung lavage fluids of normal subjects but was readily identified by means of crossed immunoelectrophoresis in CF lung fluids (n = 9, mean 49% of C3); the mean level of C3 was decreased in CF lung specimens. Chemotactic activity was significantly elevated in lung fluids of the CF patients when compared to normal lung fluids. Using gel-filtration chromatography and a sensitive radioimmunoassay the chemotaxin present in CF specimens was identified as the anaphylatoxin C5a. C5a levels in the bronchial-alveolar lavage fluids of CF patients was inversely related to volume in liters expired in 1 s of a forced expiratory maneuver expressed as a percent of vital capacity determined from a forced expiratory maneuver (r = -0.72). Because there was a direct relationship between the total elastolytic activity present in CF airways and the concentration of C5a (r = 0.97, p = 0.03), it was postulated that airway proteases with elastolytic activity also cleave C5, nonimmunologically producing C5a. Detailed inhibition assays revealed that much of the total elastolytic activity had the inhibition profile of a serine proteinase. The levels of the serine proteinases were closely correlated with the numbers of neutrophilic leukocytes present per ml of lavage fluid (r = 0.7, p = 0.05). However, inhibitors of leukocyte serine proteases did not prevent the generation of additional chemotactic activity and the proteolysis of radiolabeled C5 substrate was not prevented by inhibitors of neutrophil elastase. Although the purified metalloelastase of Pseudomonas aeruginosa was active on cell-bound and free C5 yielding C5a, inhibition of this bacterial protease in CF lung fluids only partially blocked cleavage of the alpha- and beta-chains of C5.(ABSTRACT TRUNCATED AT 400 WORDS)