The aim of this study was to assess alterations of zeta chain expression and their relation to apoptosis of T lymphocytes. T lymphocytes were obtained from malignant pleural effusions (MPE) of 15 patients. The work focused on TCR-zeta chain expression, apoptosis of T lymphocytes, and on the content of monocyte/macrophages in effusions. Analysis was performed using three color flow cytometry combining CD3, TCR-zeta and TUNEL reaction. The content of tumor and monocyte/macrophage cells has been determined using CD3, CD14, and cytokeratins, as markers of distinct cell subpopulations. Our findings strongly indicate that decreased zeta chain expression in T cells depends on the content of monocyte/macrophage cells, and that the range of the decrease is inversely proportional to the number of monocytes/macrophages in the effusion. Those having low zeta chain expression were the main subpopulation of T cells undergoing apoptosis. These data suggest that decreased zeta chain expression in T cells in MPE may be related to the abundance of monocyte/macrophages in effusions.