Expression of the Fc receptor for IgE (Fc epsilon R) was analyzed on alveolar macrophages (AM) in 10 patients with extrinsic allergic alveolitis (EAA) compared with 10 patients with sarcoidosis and to 6 apparently healthy controls. By using the anti-Fc epsilon RII mAb M-L25 in immunocytochemistry experiments, we found that greater than 60% of AM in 10 of 10 patients with EAA were strongly positive, as evidenced by visual analysis in light microscopy and by cytometry. By contrast, no significant staining was detected in sarcoidosis or in controls with either method. Similar results were obtained when Fc epsilon R were identified with preformed immune complexes consisting of NIP-specific human/mouse chimeric IgE antibody plus NIP-ovalbumin. Furthermore, greater than 60% of AM in patients with EAA stained positive for IgE, demonstrating that endogenous IgE is bound to the AM. Our data suggest that IgE antibodies bound to Fc epsilon RII on AM may be involved in pathophysiology of extrinsic allergic alveolitis by activation of the AM after binding of allergen to the cell surface IgE. Furthermore, with the clearcut pattern of Fc epsilon RII expression in extrinsic allergic alveolitis it may be possible to use CD23 antibodies for differential diagnosis of inflammatory lung disease.