Tumour necrosis factor (TNF) and interleukin-1 (IL-1) are powerful mediators with a key role in inflammation. This study was undertaken to study the presence of TNF and IL-1 in tuberculous effusion where there is marked inflammation and where examination of the pleural fluid may give information about the local inflammatory reaction. Adenosine deaminase activity (ADA, a marker of TB pleurisy) was also tested. Tumour necrosis factor, IL-1 and ADA levels were measured in the pleural fluid and serum of 97 patients; 33 with tuberculous effusion, 33 with malignant effusion, and 31 patients with benign non-tuberculous effusion. Pleural fluid TNF and ADA levels were higher in tuberculous (TB) patients than in patients with benign disorders or cancer (P < 0.01). Serum TNF levels were also higher in TB patients than other benign (P < 0.01) or malignant (P < 0.05) effusions. There was a positive correlation between serum and pleural fluid values (r = 0.998-0.999, P < 0.001) although pleural fluid concentration was higher (P < 0.001), possibly suggesting local production in the pleural cavity. Pleural fluid IL-1 levels were not raised in any patient group but there was a positive correlation between TNF and IL-1. In addition, a positive correlation was found between TNF and ADA levels, probably indicating some common production mechanism. Furthermore, ADA sensitivity in the diagnosis of tuberculous effusion was augmented by the combined use of TNF and ADA. The use of both these markers may prove useful in the differential diagnosis of TBC pleurisy.