The quantitative and qualitative characteristics of cells in the peritoneal dialysate from 12 patients were examined. The number of cells in each subsequent fraction of dialysate decreased, while the differential cell count remained relatively constant for each individual patient. Macrophages, lymphocytes, granulocytes and occasionally mesothelial cells were observed. In 1 patient, plasmocytes were also found. Evident differences in cellularity and cell composition were noticed in dialysate obtained from different patients, especially in 2 patients with bacterial peritonitis there was a rise in cellularity with neutrophilia. Cytochemical (peroxidase, nonspecific esterase activity) and functional (phagocytosis, receptor expression) tests revealed that macrophages form a heterogeneous population of cells.