The characteristics and function of lymphocytes in both pleural exudate and peripheral blood in 18 patients with tuberculous pleurisy were studied. The pleural fluid of 13 of 16 patients with tuberculous pleurisy had more T-lymphocytes than the peripheral blood. The difference between them was significant by the t test (p less than 0.05). When lymphocytes in peripheral blood were cocultured with purified protein derivative (PPD), lymphocytes from only 4 of 18 patients produced immune interferon on the fifth day. The titer was only 4 or 8 units/ml. On the contrary, lymphocytes in pleural fluid from 17 of 18 patients reacted to PPD and produced immune interferon on the fifth day, and titers were much higher (more than 128 units/ml) than those of lymphocytes in peripheral blood. When the relationship between the interferon titer produced by lymphocytes in pleural effusion and tuberculin skin reaction was investigated, the group with the stronger skin reactions showed a significantly higher interferon production (p less than 0.05). In conclusion, exudative-sensitized lymphocytes in morbid sites reacted to the specific antigen more effectively and produced titers of lymphokines than circulating lymphocytes.