A single ip injection of distilled water osmotically disrupts almost the entire population of peritoneal mast cells in rats. The metachromatic granules released from disrupted mast cells are phagocytosed by peritoneal macrophages increasing their chemotactic and spreading activities. On this basis a study was carried out to determine whether an ip injection of distilled water, by releasing an abundance of these granules for peritoneal macrophage stimulation, protects rats subsequently exposed to peritonitis. We found that a single ip injection of distilled water lowers the mortality in rats exposed to bacterial peritonitis 2-3 weeks later from 80 to 33%.