Malignant effusions in serous cavities represent suspension of tumors and inflammatory cells and therefore present an attractive model for studying tumor-host interactions. T cell functional ability is linked to proper signal transduction via T cell receptor (TCR-CD3 complex), especially its zeta chain. The latter has been found to be often depressed in malignancy. Aim of this study was to assess alterations of zeta chain expression and its relation to apoptosis of T lymphocytes. T lymphocytes obtained from pleural effusions of 20 cancer patients and 9 patients with nonmalignant effusions serving as a control were studied. The analysis was performed using flow cytometry combining CD3, CD 4, CD8, CD 16/56, TCR-zeta and TUNEL reaction. In the control group, T lymphocytes obtained from pure lymphocyte effusions had constantly high level of zeta chain while those from malignant pleural effusions had a decreased zeta chain expression. Lymphocytes with low zeta chain expression were the main subpopulation of T cells undergoing apoptosis.