Adenosine deaminase (ADA) is an enzyme which is essential for the differentiation of lymphoid cells, especially T-cells and ADA plays a role in the maturation of monocyte to macrophage. Therefore ADA levels are related to stimulation of cellular immunity. We have investigated the measurement of ADA activity in bronchoalveolar lavage fluid of the patients with active and inactive pulmonary tuberculosis and control group. The results obtained are as follows: 1) The ADA activity and corrected ADA activity from the BAL fluid in active tuberculosis group (Total Lavage ADA; $18.4{pm}22.5;mU$, Total Lavage ADA/Albumin; $2.45{pm}1.61;mU/mg$) were increased when compared with those in inactive tuberculosis (TL-ADA; $5.8{pm}2.5;mU$, TL-ADA/Alb; $1.83{pm}O.53;mU/mg$) and control (TL-ADA; $6.6{pm}4.3;mU$, TL-ADA/Alb; $1.62{pm}0.60;mU/mg$) groups. 2) The ADA activity and lavage ADA/serum ADA activity ratio in BAL fluid from the lesion site (TL-ADA; $42.9{pm}42.3;mU$, L-ADA/S-ADA; $0.53{pm}0.32$) were increased when compared with those from the non-lesion site (TL-ADA; $12.5{pm}11.2;mU$, L-ADA/S-ADA; $0.29{pm}0.12$)and normal side (TL-ADA; $12.7{pm}11.0;mU$, L-ADA/S-ADA; $0.34{pm}0.27$) in active tuberculosis group. 3) The ADA activity in BAL fluid from far advanced group (TL-ADA; $62.5{pm}30.3;mU$) was increased when compared with those from the mild group (TL-ADA; $10.5{pm}7.5;mU$) and moderate advanced group (TL-ADA; $13.2{pm}11.7;mU$) in active tuberculosis. 4) The albumin level from the BAL fluid was correlated with the ADA activity (R=0.89). 5) The ADA activity recovered from the BAL fluid was correlated with the recovered lymphocyte percentage (R=0.60). In conclusion, the ADA activity from the BAL fluid in active tuberculosis group was increased when compared with that in inactive tuberculosis and control groups, especially from the lesion site. To evaluated the specificity of ADA determination for diagnosis of active tuberculosis, BAL must be done at lesion site of the diseased lung and the proper correcting material other than albumin must be chosen to correct the dilution factor of lavage fluid.