It remains to be determined whether the peritoneal dialysis procedure induces abnormalities in the normal host defenses of the abdominal cavity and whether these perturbations are important in the pathogenesis of peritonitis. The peritoneum is a smooth membrane that lines the abdominal cavity and participates in the diffusion of water and solutes during peritoneal dialysis. The diaphragmatic lymphatic uptake and the opsonization of micro-organisms, with phagocytosis and killing by peritoneal macrophages, mesothelial cells, lymphocytes, polymorphonuclear leukocytes, and newly defined proteins such as defensins, play a combined role in the peritoneal host defense. Because the composition of earlier peritoneal dialysis fluids is clearly non-physiologic, continuous exposure of peritoneal cells to these solutions may result in an impairment of the local peritoneal host defense mechanisms. However, with the newer solutions, it has been shown that peritoneal defense mechanisms may improve.