Cathepsin B-like activity was determined in alveolar macrophages (AM) and cell-free bronchoalveolar lavage fluid (BALF) obtained from volunteers who were current cigarette smokers and compared with that found in lifetime nonsmokers. Enzyme activity was determined with benzyloxycarbonyl-Leu-Leu-Arg-2-naphthylamide as the substrate. Specific activity of the enzyme was more than twice as high in AM from smokers than in cells from nonsmokers (35,600 +/- 2,250 versus 16,000 +/- 860; p less than 0.001) and about 10 times as high in BALF from smokers than in that from nonsmokers (3,060 +/- 380 versus 300 +/- 25; p less than 0.001). Because cathepsin B is capable of degrading structural lung proteins and inactivating alpha-1-proteinase inhibitor, and the elastinolytic activity of AM may be mediated through cysteine proteinases such as cathepsin B, the finding of high concentrations of an enzyme with cathepsin B-like activity in AM and BALF from smokers suggests the need to explore the role of the enzyme in structural lung damage associated with cigarette smoking.