We have examined the distribution of B and T lymphocytes, T-cells with helper/inducer (T4+) or suppressor/cytotoxic (T8+) phenotypes and a subset of cells with natural killer (NK) activity and positive for the Leu 7 (HNK-1) surface antigen in peripheral blood and in lymphocyte-rich pleural effusions of patients with tuberculosis or malignancies (mesothelioma and lung cancer with pleural metastasis). In individual patients, the percentages of T lymphocytes were uniformly higher in pleural effusions than in peripheral blood; however, lower percentages of B lymphocytes and cells positive for the Leu 7 antigen were present in pleural fluids. The analysis of T-cell subpopulations demonstrated a selective enrichment of T lymphocytes with helper/inducer phenotype in pleural effusions, while the percentages of T-cells with suppressor/cytotoxic phenotype were similar in pleural fluid and peripheral blood. These results indicate that in lymphocytic pleural effusions the main lymphoid cell population is represented by T lymphocytes with helper/inducer phenotype, regardless of whether the effusion is due to tuberculosis or malignancies such as mesothelioma or lung cancer.