The contradictory data in the literature in the study of the adenosine deaminase (ADA) in serum for the differential diagnosis of infiltrative pulmonary tuberculosis and pneumonia were the basis for this study. The ADA and its isoenzymes in the blood and mononuclear was studied in 80 patients with infiltrative pulmonary tuberculosis (IPT) and 17 - pneumonia (P). Mycobacterium tuberculosis was isolated from 84.0% of patients IPT. Total serum ADA activity was identical in both increased diagnoses. Increase in serum ADA activity is mainly due to its ADА-2 isoenzyme. In patients with the ITL, as compared to the reference values, showed a reduction in the activity of ADА-1 and growth of ADА-2, and as a consequence, the ratio of isoenzymes in overall activity of the ADA was vio- lation: decrease the percentage of ADА-1 and an increase in ADА-2. Patients with PV activity of ADА-1 and ADА-2 were defined within the reference range, and maintained dynamic balance ADA isoenzymes. In patients with ITL fraction isoenzyme ADА-1 in the total activity ADA was lower and ADА-2, in contrast, higher than with PV, but the frequency of these parameters is lower / higher X±σ did not differ between groups. The mononuclear cells in both diagnoses, compared with the reference value,decreased activity ADAmn by reducing the activity of ADА-1mn while maintaining within the reference range of the activity of ADА-2mn and dynamic balance ADAmn isoenzymes. Patients with PV ADА-2mn activity and percentage ADА-2mn isoenzyme in general activity ADAmn identified below, and the proportion of ADА-1mn contrast, higher than the ITL patients, but these figures are within the reference range. Thus, the activity of ADA and its enzymes in serum and mononuclear cells can not be used in differential diagnosis of newly diagnosed infiltrative pulmonary tuberculosis and pneumonia.