In 63 patients with chronic bronchitis, 61 patients with acute bacterial pneumonia, and in 11 control subjects, broncho-alveolar lavage fluid was investigated for its protease content. In smokers, we found the usual pronounced protease burden. As was expected, the two pathological groups also differed markedly on other points. In pneumonia, not only free elastase activity (from granulocytes (PMN] was markedly elevated, but also cathepsin-B-like activity (from alveolar macrophages (AM] was greatly increased, which would point up the important role of AM in this form of inflammatory. In this case of chronic bronchitis, function-analytically determinable obstruction parameters were associated with an increase in the elastase-like enzyme (from PMN), so that chronic obstructive bronchitis probably represents a special form of inflammation.