The immune function of peripheral blood cells and cells from the pleural and abdominal effusions of patients with advanced cancer was compared to that of peripheral blood cells from controls. The parameters examined included lymphocyte subsets, natural killer (NK) cell activity, and anti-Daudi and lymphokine-activated killer (LAK) cell activity. The percentage of CD4+ pleural and peritoneal exudate cells (PEC) was significantly higher than the percentage of peripheral blood mononuclear cells (PBMC) in the patients. The percentage of CD8+CD11+ PEC and PBMC, being the suppressor T-cells, of the patients was increased compared with controls, while the percentage of CD8+CD11- PEC, being the cytotoxic T-cells, was identical to the PBMC of both patients and controls. The NK activity of PEC was significantly lower than that of PBMC in both patients and controls, and there was no correlation between the NK activity of PBMC and PEC. Although the anti-Daudi activity of PEC was markedly low, LAK cells with high activity could be induced by culture with interleukin-2 for 4 days. These results suggest that the immune function of cells in malignant effusions may be depressed due to a low population of cytotoxic T cells, low NK activity and increased suppressor T cells, while the local administration of interleukin-2 may induce LAK cells. Therefore, effective local immunotherapy for malignant effusions should not only augment effector cells, but also inhibit suppressor cells.