Peroxidase activity was assessed cytochemically in macrophages from peritoneal dialysate of 50 patients with terminal renal failure treated by intermittent peritoneal dialysis. In 22 patients cells were harvested for the second time after development of peritonitis as the complication of the treatment. The obtained results were compared with those of the control group consisting of 30 patients with normal renal function. Patients treated with peritoneal dialysis showed both in complication-free period and during the course of peritonitis significantly higher peroxidase activity in macrophages as revealed by the semiquantitative score test. In the course of peritonitis, peroxidase activity in macrophages was significantly lower than in complication-free period. Electron-microscopic cytochemistry demonstrated that in macrophages obtained from both control and dialyzed patients the peroxidase activity was located exclusively in cytoplasmic granules, suggesting a similarity of the studied cells and exudative macrophages.