This study attempts to correlate levels of ADA in tuberculous and neoplastic pleural exudates with the different immunologic cellular expressions that follow these clinical situations. Seventy-three patients with pleural effusion were studied in order to assess ADA activity (pleural and serum); in 25 of them, a study of delayed cellular immunity (pleural and sanguineous) was performed through B, CD3, CD4, and CD8 lymphocytic populations. The activity of ADA was determined, and the study of lymphocytic populations was made through the use of monoclonal antibodies. The data obtained showed the following: levels of ADA were significantly (p less than 0.0005) higher in the pleural fluid and the serum of tuberculous effusions compared to neoplastic effusions; percentages of CD3 and CD4 T-cells were significantly (p less than 0.05 and p less than 0.0005, respectively) greater in tuberculous effusions. The statistical study of the levels of ADA activity and the percentage of CD4 T-cells in pleural exudates produced a significant regression curve (r = 0.612 and p less than 0.0001) which showed a positive correlation between these two parameters. The pathogenic implications of these results suggest the possibility that ADA could be a new marker of cell-mediated immune activity.