The aims of this study were to develop a methodology for the isolation of highly enriched mononuclear phagocyte populations from exudative malignant pleural effusions (EMPE) and to characterize the phenotype and functional properties of these cells. Pleural effusion mononuclear cells (PEMC) were isolated by Ficoll centrifugation of EMPE and transudative pleural effusions and allowed to adhere to plastic for 1 h to obtain a pleural effusion mononuclear adherent cell (PEMAC) fraction. Only 66.0 +/- 4.2 percent of PEMAC ingested latex particles, indicating that a significant proportion of PEMAC were not phagocytic cells. Latex-positive PEMAC had the morphologic appearance of macrophages and stained positive (97.3 +/- 4.3 percent) with the anti-CD68 monoclonal antibody (MoAb), specific for macrophages. Conversely, latex-negative PEMAC (34.0 +/- 4.1 percent of PEMAC) did not react with the anti-CD68 MoAb and stained with anti-CD3 (34.7 +/- 10.7 percent) and anticytokeratin (50.5 +/- 16.4 percent) MoAbs, indicating that T cells and mesothelial cells were present in the PEMAC fraction. To improve the purification of pleural macrophages, PEMAC were cultured for an additional 18 h and the cells that remained adherent after this period constituted the firmly adherent mononuclear cell (FAMC) fraction. Nearly 90 percent of FAMC ingested latex particles and were CD68-positive. Virtually all FAMC were CD3-negative and cytokeratin-negative. Similar percentages of FAMC from EMPE and transudative effusions expressed the monocyte-lineage markers CD11b and CD14, suggesting that the proportion of monocyte-like mononuclear phagocytes in the pleural space is not increased during local tumor-associated inflammatory responses. The FAMC from EMPE (1) expressed HLA-DR antigens, (2) released interleukin 1 (IL-1) beta and tumor necrosis factor (TNF) alpha, and (3) stimulated allogeneic T-lymphocyte proliferation. The results of this study suggest that pleural mononuclear phagocytes may be involved in tumor-associated inflammatory reactions in the pleural compartment by stimulating the proliferation of other inflammatory cells and by releasing inflammatory cytokines.