Acid phosphatase (AcP), beta-glucuronidase (GR) and N-acetyl-beta-D-glucosaminidase (NAG) activity was determined, using semiquantitative cytochemical methods, in the peritoneal fluid lymphocytes obtained from 50 patients with terminal renal failure treated by intermittent peritoneal dialysis. The control group included 30 subjects with normal renal function. The percentage of AcP and NAG-positive lymphocytes was significantly lower and that of the GR-positive cells significantly higher in dialysed patients than in the control group. A group of 22 dialysed patients with bacterial peritonitis showed a significant increase of the percentage of NAG-positive lymphocytes as compared with both the subjects in the control group and the peritonitis-free dialysed ones. Changes of the lymphocytes enzymatic activities were distinct in cells exhibiting the granular reaction type, and to a much lesser extent in those showing granular diffuse reaction.