Lymphoid cells, isolated from malignant pleural effusions and collected from patients bearing primary lung carcinoma, were examined by means of indirect immunofluorescence and a panel of monoclonal antibodies vs several CD antigens. The percentages of CD4+ T lymphocytes were found to be significantly depressed in malignant effusions as compared to inflammatory ones. In relation to histological type of cancer it was especially evident in squamous cell and anaplastic carcinomas (small and large cell), in comparison to adenocarcinomas. Expression of T cell antigen receptor tau/delta (TCR-1) on T lymphocytes, demonstrated by BB3 MoAb (vs V delta 2), was significantly higher in malignant effusions as compared with non-malignant ones. This was not the case when A13 MoAb (equivalent of TCS 1) was used (vs V delta 1). Percentage values of NK cells, monocyte/granulocyte series activated cells and B lymphocytes did not differ significantly in malignant and non-malignant effusions. It is concluded that these are T lymphocyte subpopulations which are apparently distinct in both effusion groups examined.