Elastase activity was measured in concentrated, cell-free bronchoalveolar lavage (BAL), using the synthetic substrate butyloxycarbonyl-L-alanyl-L-alanyl-L-prolyl-L-valyl-amino-methylcoumarin. The BAL fluids obtained from young, asymptomatic smokers with normal urine desmosine concentrations 1 h after they had smoked 2 cigarettes showed significant increases in elastase levels compared with those in nonsmoking control subjects [nanomoles substrate hydrolyzed (3 h) per milligram lavage albumin = mean 2.7 +/- 1.9 SD (11 smokers) versus 0.5 +/- 0.4 (11 nonsmokers), p less than 0.01]. Repeated BAL samples were obtained at later times from one smoker with a high initial enzyme value and from one nonsmoking control subject. Elastase activity varied over time, but both subjects consistently remained within their respective group ranges. Inhibition studies on pooled BAL from smokers showed that the elastase activity present had properties of both serine and metalloenzymes, suggesting that neutrophils and/or monocytes (serine enzyme) as well as macrophages (metalloenzyme) contributed to the observed activity. Lung lavage cells obtained from 2 of the smokers and 2 of the nonsmokers were stained with both a chromogenic substrate and by indirect immunofluorescence for the serine enzyme. Positively stained neutrophils were readily found in smokers' lavages, but no, or only rare, positive mononuclear cells could be identified. By contrast, peripheral blood mononuclear cells from all 4 subjects stained positively with either method. These results show that some asymptomatic smokers have significantly more elastase activity in their bronchopulmonary secretions than do nonsmokers (as measured with a low molecular weight synthetic substrate). Furthermore, the enzyme activity recovered in smokers' BAL appears to be derived mainly from neutrophils (serine enzyme) and macrophages (metalloenzyme), rather than from monocytes.