Our project identified, by flow cytometry, the immunophenotypes and activation state of the immune cells in the peritoneal dialysis fluid from patients with peritonitis on continuous ambulatory peritoneal dialysis. The results showed that all kinds of cells of the immune system were present in the peritoneal fluid in percentages and activation states similar to those seen in blood. Also, two subgroups of patients were noted, according to CD4:CD8 ratio. Patients whose ratio was < 1.1 had more expansion of CD8 and NK cells, and a higher percentage of B1 cells in both fluids than were seen in healthy people.