IT WAS PREVIOUSLY FOUND (1) that insulin can bedetected in human blood by injecting it intraperitoneally into the hypophysectomized adreno demedullated rat. If 1 cc.of human blood wasinjected per 1oo gm.of rat the blood sugar fell from 58 to 51 mg.% indicating an insulin content of 0.0002 U ofinsulin percc. In order to determine the specificity ofthis reaction forinsulin, experiments were carried out on pancreatectomized dogs. They were supplemented by studies ontheinfluence of insulin injected into these animals on the insulin content of the blood. The blood tobe tested for insulin wasinjected immediately into the peritoneal cavity of the hypophysectomized adreno demedullated rat. The procedure was the same as reported previously, except that 4 cc. of blood per 100 gm. rat were injected. The blood was tolerated bythe animals without any ill effects. Table 1 shows the hypoglycemic reaction produced bythe injection of normal dog's blood intothe hypophysectomized-adreno-demedullated rat.It wasfound that the blood sugar inthe test animal waslowered from 60 to 48 mg.%, which corresponds to approximately 0.0001 U of insulin per cc.of blood.