The activity of adenosine deaminase has been measured in 295 specimens of pleural fluid from 248 patients. The effusions were due in part to pleural tubercle (n = 8), rheumatoid arthritis (n = 2) empyema (n = 4), or malignant lymphoma (n = 5); the remainder were due to effusions of other aetiologies (n = 229). The disorders of the first group of patients are known to be associated with an elevated level of ADA. The two groups of patients were compared by fixing the upper limit of normal at 50 U/l. There was a 97% specificity even though the sensitivity was only 42%. However the relative smallness of the group of patients who were suffering from tuberculosis, rheumatoid arthritis, empyema and malignant lymphoma means that the interpretation of the sensitivity of the test should be subject to caution.