Sarcoidosis and hypersensitivity pneumonitis (HP) are two granulomatous pulmonary diseases, the former of unknown cause and the latter with known etiology. To compare these diseases, we practiced bronchoalveolar lavage on 16 patients with sarcoidosis, 6 with HP, 10 healthy nonsmokers, and 10 smokers with chronic bronchitis. Lymphocyte subpopulations were evaluated by four membrane markers: (1) The total cell count was very high in patients with HP compared with those with sarcoidosis and with healthy subjects. (2) The proportion of lymphocytes was higher in patients with HP than in those with sarcoidosis (P less than 0.02) and also higher in patients with sarcoidosis than in healthy subjects (P less than 0.001). (3) There were only a small number of B cells and of Fc-receptor bearing cells. (4) The lymphocytes identified by our techniques were mainly T cells in sarcoidosis and HP (68.4 +/- 3.5 percent and 76.8 +/- 3.6 percent, respectively), but the proportion of "active" E-rosette-forming cells was higher in sarcoidosis than in HP (28.1 +/- 2.5 percent vs 9.8 +/- 1.5 percent) and higher in patients with HP than in healthy subjects. These studies suggest that: (1) these two granulomatoses may be differentiated by this method, which might also be used for other pulmonary angiitis and granulomas; (2) cell-mediated immune phenomena play a major role in the pathogenesis of sarcoidosis, and conversely the finding of low active E-rosette proportions in HP limits the role of delayed-type hypersensitivity; and (3) the exact role of immunologic activation of bronchopulmonary macrophages, "presenting" antigen to T cells, for instance, must be evaluated.