Cellular and immunoglobulin components of bronchoalveolar fluid recovered by bronchoscopic lavage were evaluated in 32 control patients, 10 normal volunteers, and 60 patients with the following interstitial lung diseases: idiopathic pulmonary fibrosis, pulmonary fibrosis associated with collagen-vascular disease, eosinophilic granuloma, sarcoidosis, and hypersensitivity pneumonitis. The percentage of lymphocytes distinguished two general disease categories: those with increased lymphocytes (sarcoidosis and hypersensitivity pneumonitis); and those with normal lymphocytes (idiopathic pulmonary fibrosis, pulmonary fibrosis associated with collagen-vascular disease, and eosinophilic granuloma). Patients in all five disease categories had elevated IgG levels and percentages of neutrophils compared with control patients, with the highest proportion of neutrophils found in idiopathic pulmonary fibrosis. Immunoglobulin levels also helped distinguish among patient groups, in that patients with hypersensitivity pneumonitis had lavage IgG/albumin ratios greater than 1, whereas patients with sarcoidosis had ratios less than 1; and with infrequent exceptions, the finding of IgM in lavage fluid was limited to patients with hypersensitivity pneumonitis.