Chemiluminescence and flow cytometry, employing a number of monoclonal antibodies, was used to investigate activation of immunocompetent cells in the blood and peritoneum of patients treated with CAPD. Increased expression of HLA-DR and RFD7, both markers of macrophage maturation, was observed on peritoneal macrophages, 78% and 33.5% cells positive, when compared to blood monocytes, which were 46% and 5.3%, respectively (P less than 0.001). Macrophage chemiluminescent response to opsonized zymosan was greater than that of circulating blood monocytes in CAPD patients, whereas the inverse was true for normal controls. Enhanced expression of IL-2 receptor and surface IgG by peritoneal macrophages were 24.9% and 65.3% cells positive compared with monocytes, 5.7% and 12.3% (P less than 0.01), and also suggests their activation. There was a marked increase in the HLA-DR expression by peritoneal lymphocytes from CAPD patients (32% cells positive) compared with those from CAPD blood (13.8%), normal blood (11.3%) and normal peritoneal fluid (14.5%), P less than 0.001. This was partially accounted for by increased numbers of B cells (18%) and activated T cells bearing HLA-DR. TFR and IL-2 receptor expression by CAPD peritoneal lymphocytes was similar to that of blood lymphocytes, implying the lack of an organized immune response within the peritoneum. Taken together, these results suggest that peritoneal macrophages from CAPD treated patients have features of maturation and activation, while changes in the lymphocyte populations are compatable with the actions of IL-1, indicating activity of the cellular immune system within the peritoneum.