The activity of adenosine deaminase (ADA) was determined in serum and pleural fluid of 90 patients with pleural effusions of various aetiology. Tuberculous pleural effusions, empyemas and rheumatoid pleural effusions demonstrated significantly higher activities of ADA than parapneumonic , nonspecific and malignant pleural effusions and effusions in systemic lupus erythematosus and congestive heart failure. In tuberculosis, empyema and rheumatoid arthritis ADA activity was significantly higher in pleural fluid than in serum, indicating a local synthesis of ADA by cells within the pleural cavity in these diseases.