The concentration of a low molecular mass bronchial mucus inhibitor of proteolytic enzymes (BMPI) was measured in lung secretions from patients with chronic bronchitis and compared to the concentrations of albumin and alpha 1-proteinase inhibitor (alpha 1-PI). The concentration of all three proteins was lower in bronchoalveolar lavage samples than in sputum obtained from the same patient. However the relationship between the proteins (concentrations relative to each other) was similar in both secretions suggesting the differences in concentrations were only dilutional. The molar concentration of BMPI in both secretions was generally greater than that of alpha 1-PI suggesting that most of the anti-elastase screen (congruent to 75%) was due to the former protein. Furthermore inhibitory studies using both purified leukocyte elastase and porcine pancreatic elastase show that the inhibitory capacity of lung alpha 1-PI varied and accounted for about 16% of the total inhibition of leukocyte elastase in sputum. However, the inhibitory function of alpha 1-PI was less in bronchoalveolar lavage fluids compared to sputum (p less than 0.05) and accounted for about 5% of the total inhibitory capacity for leukocyte elastase. It is concluded that alpha 1-PI contributes only a part of the anti-elastase screen of secretions from patients with chronic bronchitis. Furthermore it is inactivated to a varying degree in most secretions and its remaining inhibitory capacity represents only a minor proportion of the total inhibitory capacity.