In the present study, the expressions of B cell activating factor belonging to the tumor necrosis factor family (BAFF) and its receptors (BAFF-R and TACI) on T lymphocytes from malignant pleural effusion (MPE) were examined by fluorescence-activated cell sorting (FACS) analysis, and compared with those on the T lymphocytes from non-malignant pleural effusion (NMPE) and healthy controls. It was found that CD3 positive T lymphocytes (including CD4, CD8, and part of CD25 and CD69 positive cells) of MPE in lung cancer highly and consistently expressed the BAFF molecule, while high expressions of BAFF could only be found in phytohemagglutinin (PHA) or interleukin 2 (IL-2) induced T lymphocytes from NMPE or healthy controls. These results were consistent with the results from BAFF mRNA detection by real-time PCR. In addition, T lymphocytes from MPE expressed significantly more BAFF-R than those from NMPE or healthy controls, while the expression of TACI was increased on CD4+ T cells but decreased on CD8+ T cells when compared with controls. The Annexin/PI assay suggested that recombinant human BAFF (rhBAFF) could promote the survival rate of T lymphocytes from MPE, while the decoy receptor TACI-Fc fusion protein could promote the apoptosis rate of T lymphocytes. Cytokines in the supeatant detected by ELISA assay showed that rhBAFF could significantly upregulate the secretion of IFN-γ in vitro,and the IFN-γ level in the TACI-Fc-treated group resembled that of the control groups. All of these results indicated that the abnormally high expression of BAFF on T lymphocytes from MPE may play a role of antitumor effect.