Abstract The effects of glucose oxidase, which oxidizes glucose and produces H 2O 2, were studied in mice after intraperitoneal injection of supralethal doses of the enzyme. Such injections cause the appearance of hematin pigment in the plasma, up to 20 per cent of this pigment being methemoglobin. After high lethal doses of glucose oxidase, the hematocrit decreases (readings as low as 7 have been obtained), and the total hematin pigment of whole blood also decreases strikingly. After lower, but stilllethal, doses of the enzyme, the hematocrit increases (readings as high as 80 have been obtained), and the percentage of hematin pigment in whole blood correspondingly increases. At least part of this loss of fluid from the blood is accounted for by increased water content of liver and kidney. After such doses of glucose oxidase, decreases were demonstrated in blood sugar and in liver glycogen.