To evaluate the diagnoostic value of ADA and lysozyme activity of serum and pleural fluid, we measured the ADA and lysozyme activity of serum and pleural effusions in 50 cases of pleural effusions due to tuberculosis (n=33), malignancy (n=13), and congestive heart failure (n=4). The results were as follows: 1) The mean value of serum ADA activity of tubercu-lous effusion was 45.533,3 U/L and was significantly higher than that of nontuberculous effusion(p<0,05). 2) The mean value of pleural ADA activity of tuber-culous effusion was 11.7±47.9 U/L and was significantly higher than that of nontuberculous effusion(p<0.005). 3) The mean value of the pleural and serum ADA ratio of tuberculous effusion was 2.9±1.4 and was significantly higher than that of nontuberculous effusion (p<0.05). 4) The serum lysozyme activity was not significant between tuberculous and nontuberculous effusions. 5) The mean value of pleural lysozyme activity of the tuberculous effusion was 18.0±10.3 μg/ml and was significantly higher than that of nontuberculous effusion. 6) The mean value of the pleural and serum lysozyme ratio of tuberculous effusion was 1.8±1.1 and was significantly higher than that of nontuberculous effusion. The ratio above 1.2 had a sensitivity of 70% and a specificity of 100% when used as a screening test for tuberculous effusion compared with malignant effusion. In conculusion, the measurements of serum and pleural ADA, lysozyme and their ratios were useful in the differential diagnosis between the tuberculous and nontuberculous pleural effusions.