In serum and sputum samples from 15 patients with cystic fibrosis (CF) suffering from chronic Pseudomonas aeruginosa lung infections, concentrations and/or activities of elastase derived from polymorphonuclear leukocytes (PMN), cathepsin G, myeloperoxidase (MPO), and superoxide dismutase (SOD), as well as concentrations of the proteinase inhibitors α1-proteinase inhibitor (α1-PI) and α2-macroglobulin (α2-M), were determined. High enzyme concentrations compared with those in normal control subjects were found for PMN elastase (mean, 96.1 ± 91.7 μg/ml), cathepsin G (mean, 5.9 ± 6.0 μg/ml), and MPO (mean, 138.0 ± 177 μg/ml) in patients' sputum samples. Superoxide dismutase was not detectable in any of the sputum specimens (below 1 ng/ml). Proteinase inhibitor concentrations were elevated in serum samples (α1-PI: mean, 3,457 ± 1,084 μg/ml; α2-M: mean, 4,835 ± 1,334 μg/ml). Means of 61 ± 38 μg/ml α1-PI and 29 ± 31 μg/ml α2-M were present in the sputum specimens. Both proteinase inhibitors were functional in the serum samples. However, sputum α1-PI was proteolytically degraded, as shown by western blot technique, and was not able to bind ¹²⁵l-labeled PMN elastase, as shown by autoradiography. Only 10.9 ± 8.5% of the total α1-PI in the sputum samples was complexed to PMN elastase and 3.6 ± 3.2% to cathepsin G. On the other hand, 96.2 ± 96.8% of the total PMN elastase and 78.0 ± 100% of cathepsin G were unbound in the sputum samples. The study suggests that the imbalance between PMN proteinases and their inhibitors is due to inactivation of α1-PI in the sputum caused by proteolytic or oxidative attack from PMN enzymes.