The levels of endotoxin in blood were determined using the Limulus lysate test in combination with a chromogenic substrate. Plasma was analyzed from four patients with fatal meningococcal septicaemia and from one patient who survived meningococcal meningitis. All septicaemia patients showed high levels of endotoxin in their blood during the early stage of their disease. In two of these patients, blood samples collected at intervals of two days revealed a gradual disappearance of measurable endotoxin from the circulation. The patient with meningitis had no clinical signs of circulatory deficiency or coagulopathy and was consistently negative for endotoxin using this test procedure. Pretreating the plasma with heat and alkali and combining the Limulus lysate test with a chromogenic substrate seem to provide a sensible method for the detection and quantitation of endotoxin in blood.