It has been shown that bronchoalveolar lavages (BALs) from patients with sarcoidosis and other interstitial lung diseases contain abnormally increased numbers of alveolar macrophages (AM) expressing antigens found on monocytes. The aim of this study was to compare the phenotype of AM from patients with sarcoidosis with those from patients with non-interstitial lung disease, namely carcinoma. Using a panel of monoclonal antibodies against cells of the mononuclear phagocytic series and immunohistochemical staining, we have compared the expression of antigens on AM recovered at BAL and peripheral blood monocytes from patients with sarcoidosis, with similar cell preparations from bronchogenic carcinoma patients and normal volunteers. We have shown that CD14, CR1 (CD35) and CR3 (CD11b, CD18) are expressed on the majority of monocytes from all subjects, but on only a minority of normal AM. In both patients with sarcoidosis and patients with bronchogenic carcinoma increased proportions of AM expressed these monocyte-associated antigens. Since BALs from the carcinoma patients were derived from lung lobes which were radiologically free of tumour, the accumulation of AM expressing monocytic antigens is not a local response to the tumour. We conclude that infiltration of the lung with monocytes is a more general response to lung disease than has hitherto been reported.