An ‘epidemic’ of aseptic peritonitis occurred in our peritoneal dialysis unit, affecting 5 of 20 patients. Acute and convalescent viral titers were normal in all 5. The peritoneal fluid of the affected patients was not tested for endotoxin, but endotoxin was found in subsequent dialysis fluids from two machines in the unit. This endotoxin might have been the causative agent of this outbreak. Rapid recovery ensued in all patients following peritoneal lavage.