Our study was undertaken to investigate the role of tumor necrosis factor-alpha (TNF-alpha) at the site of disease activity in tuberculous pleuritis (TP). Concentrations of TNF-alpha were measured directly by radioimmunoassay (RIA) in the pleural fluid (PF) and plasma (P) of patients with TP (n = 14), malignant effusions (n = 18) and transudates (n = 7). Among these three groups mean plasma levels of TNF-alpha were not statistically significant (P > 0.05), but in the TP group mean levels of TNF-alpha in PF were significantly higher than in the cancer and transudate groups (P < 0.001), and there was no significant difference between the cancer and transudate groups (P > 0.05). These results may indicate a local synthesis of TNF-alpha by cells within the pleural cavity in this disease.