Abstract： Objectives　To explore the significance of TNF, IL-8, sICAM-1 and CD11b/CD18 in pleural effusions and/or in peripheral blood in formation of pleural effusions and the possible role in differential diagnosis. Methods　Levels of TNF, IL-8, sICAM-1 and PMN CD11b/CD18 expression were measured by radioimmunoassay (RIA), enzyme-linked immunosorbent assay (ELISA) and flow cytometry (FCM). 31 patients with tuberculous pleural effusions, 31 malignant pleural effusions and 31 healthy persons as control were studied.Results　The serum levels of TNF, IL-8 and PMN CD11b/CD18 expression in benign and malignant effusions were markedly higher than those in controls (P<0.01). The levels of TNF, IL-8 and PMN CD11b/CD18 in patients with tuberculous pleural effusions were elevated higher than those with malignant effusions (P<0.01). The levels of sICAM-1 in pleural effusions were much lower in tuberculosis than malignancy (P<0.01). TNF levels in pleural effusions were positively correlated to the IL-8 and sICAM-1 levels (r=0.74 and 0.79, respectively, P<0.01). TNF levels in serum was positively correlated to the PMN CD11b/CD18 expression (r=0.61, P<0.01), but the latter was negatively correlated to sICAM-1 levels of pleural effusions (Y=1442.31-36.85X+0.25X2，R2=0.59，F=19.83，P<0.01). Conclusions　The results show that TNF, IL-8, sICAM-1 and CD11b/CD18 may work and affect each other in immunopathological process of tuberculous and malignant pleural effusions. The changes of TNF, IL-8 in pleural effusions and the expressions of PMN CD11b/CD18 are of clinically diagnostic value in distinguishing tuberculous from malignant pleural effusions.