Selected proteins were compared in bronchial lavage fluids recovered from 36 healthy nonsmokers and 19 healthy cigarette smokers. Although fluids lavaged from smokers contained 4 times as many cells (more than 80 per cent macrophages) as fluids from nonsmokers, there were no quantitative differences between the two groups in immunoglobulin A, a1-antitrypsin, a2-globulin, transferrin, or albumin content. However, immunoglobulin G content of bronchial fluids from smokers was twice as high as that from nonsmokers. This selective increase in immunoglobulin is compatible either with stimulated local bronchial immunoglobulin production or the selective exudation of plasma immunoglobulin into alveolar spaces in response to inhaled cigarette smoke.