The characteristics of antigen-presenting cells in carcinomas that involve the abdominopelvic cavity are unknown. Dendritic cells, a population of antigen-presenting cells, have been identified as lineage-negative human leukocyte antigen (HLA)-DR+ cells by two-color flow cytometry. We used this criterion to study the putative dendritic cells in ascites from 25 patients with peritoneal carcinomatosis. The mean proportion +/- SD of lineage-negative HLA-DR+ cells in ascites was 3.1 +/- 4.6% (range, 0.05-17.3%). Most lineage-negative HLA-DR+ cells expressed CD45RA or CD4 antigens. Dendritic cells had low proportions of CD80, CD11c, CD45RO, and CD58, suggesting that they were of low maturity. The proportion of lineage-negative HLA-DR+ cells in ascites of seven patients was significantly higher than the proportion in peripheral blood from the identical patients (4.5 +/- 5.7 versus 0.5 +/- 0.4; P < 0.05). In paired specimens of ascites and peripheral blood, the proportion of lineage-negative HLA-DR+ cells that coexpressed CD86 or CD58 was significantly lower in ascites than in peripheral blood, whereas a higher proportion of lineage-negative HLA-DR+ cells in ascites expressed CD4. Relative fluorescence intensity of HLA-DR+ was also lower in dendritic cells from ascites and blood from patients with carcinomatosis than it was in blood from normal donors. As an indicator of macrophage activation, the concentration of neopterin in ascitic fluid correlated negatively with the numbers of lineage-negative HLA-DR+ cells in ascites (Spearman correlation coefficient, -0.44; P = 0.05) correlated positively with the concentration of interleukin 10 in ascitic fluid (Spearman correlation coefficient, -0.40; P = 0.05). IFN-gamma and tumor necrosis factor alpha were also not detected. These findings suggest that certain factors associated with the tumor microenvironment might influence the number of these dendritic cells and their expression of function-associated markers.