Simultaneous determination of ascitic fluid and serum adenosine deaminase (ADA) activity was evaluated as a diagnostic aid in peritoneal tuberculosis. The ascites was due to peritoneal tuberculosis (group 1), cirrhosis of the liver (group 2), cirrhosis of the liver with spontaneous bacterial peritonitis (group 3), peritoneal malignancy (group 4), Budd-Chiari Syndrome (group 5) and miscellaneous conditions (group 6). Serum from patients of pulmonary tuberculosis and healthy volunteers was analysed for enzyme activity. In patients with peritoneal tuberculosis the ascitic fluid and serum ADA activity was significantly higher than for the other groups (P less than 0.001). Levels above 36 u/l in ascitic fluid and above 54 u/l in the serum suggest tuberculosis. The ascitic fluid/serum ADA ratio was also higher in patients with peritoneal tuberculosis than with other causes of ascites (P less than 0.01). A ratio of more than 0.984 was suggestive of tuberculosis.