The catalytic concentration of pleural adenosine deaminase (ADA) and the ratio of pleural lysozyme (PL) to serum lysozyme (SL) were measured in consecutive patients (49 tuberculous and 179 nontuberculous) with two automated procedures in a Hitachi 717 analyzer. Using sensitivity and specificity curves, we established cutoff values at 33 U/L for ADA and 1.7 for the PL/SL ratio. The sensitivity of ADA activities for tuberculous effusion was 90%, specificity 85%. Combining ADA with the PL/SL ratio enhanced specificity to 99%. However, high values for ADA and lysozyme ratios are not, alone or in combination, sensitive or specific enough to replace pleural biopsy or culture of pleural fluid for the diagnosis of tuberculous empyema.