Malignant cells isolated from 26 malignant pleural effusions, collected from patients bearing metastatic carcinoma, were examined using flow cytometry. Expression of Ki-67 antigen, DNA ploidy, S phase fraction, DNA index of tumor cells and the percentage of aneuploid cell populations were determined. Lymphoid cells from the same malignant effusions and from 10 nonmalignant ones were examined using a panel of monoclonal antibodies (mAb) versus several CD antigens. While the percentages of T cells (CD3+, CD4+, CD8+) and B cells (CD19+) did not differ significantly in malignant versus nonmalignant effusions, those of CD56+ lymphocytes were found to be significantly depressed in malignant diploid and malignant aneuploid effusions as compared to benign ones. Expression of IL-2 receptors on lymphocytes, demonstrated by anti-CD25 mAb, was significantly higher in malignant diploid and malignant aneuploid effusions as compared with nonmalignant ones. An increase in the percentage of Ki-67-positive malignant cells was significantly correlated with the decrease of NK cells. S phase fraction, DNA index and the percentage of aneuploid populations did not correlate with the picture of lymphoid cells from pleural effusion. On the other hand, the percentage of Ki-67-positive tumor cells in pleural effusions was inversely proportional to the patients' survival time, although it was not statistically significant. Our data suggest that the poor prognosis of cancer characterized by the presence of an aneuploid cell population of malignant cells and the high percentage of proliferating cells in pleural effusions can be linked to the depression of NK cells.