Cytospin preparations of mononuclear inflammatory cells were made from bronchoalveolar lavage fluid obtained from 15 patients with interstitial lung disease (nine patients with sarcoidosis and six patients with cryptogenic fibrosing alveolitis) and six control subjects. These preparations were examined with a panel of monoclonal antibodies that have been shown to distinguish subpopulations of macrophage like cells in normal tissues. The lysosomal acid phosphatase activity of the cells was also assessed. Phenotypically distinct subpopulations of alveolar macrophages were identified in all samples studied. The results showed that all cell populations identified in bronchoalveolar lavage fluid from the groups with interstitial lung disease could be identified in the lavage fluid from normal volunteers. Some quantitative differences in the proportions of cells identified with particular reagents emerged. In each of the groups with interstitial lung disease increased proportions of cells were identified with RFD1 (interdigitating cell marker; p less than 0.01) and in the cryptogenic fibrosing alveolitis group an increased proportion of alveolar macrophages was identified with RFD7 (tissue macrophage marker; p less than 0.05). The possibility that quantitative changes in alveolar macrophage subsets observed in the interstitial lung disease groups are relevant to the pathogenesis of these conditions is discussed.