It must not be inferred from the tabulated reactions that a transfusion with the blood of an incompatible group necessarily produces a fatal, or even a serious, result. If, for instance, an individual of Group II be transfused with blood of Group III, the corpuscles of the donor’s blood will certainly be rendered ineffective, being destroyed either at once or in the course of a short time. But beyond this wastage of the transfused blood there may be no effects as shown by morbid symptoms in the recipient; he will merely not be benefited. There may, on the other hand, be an evident reaction in the recipient, the symptoms varying from slight discomfort to almost immediate death. It appears, therefore, that there is a gradation of toxicity between the bloods of incompatible groups, so that it may be justifiable owing to extreme urgency in certain cases to perform a transfusion without doing any preliminary tests on the bloods of donor and recipient. There is a good chance that the groups will be compatible; if, however, they be incompatible, there is still a good chance that the recipient will be no worse off than he was before the transfusion. Even when the tests have been performed, it may still happen that through various causes a mistake has arisen. Owing to the inexperience of the operator or to staleness of the sera used in performing the test, an incompatible group may appear to be compatible. It is necessary, therefore, that everyone who performs a transfusion should be able to recognize the symptoms of a reaction as soon as it begins to appear, so that the transfusion may be at once discontinued. Sometimes the reaction between incompatible groups is so immediate and severe that death takes place almost at once. I did not myself perform any transfusions until after the period when blood-grouping tests had become a routine procedure, so that I have no personal experience of such unfortunate results. The symptoms may therefore best be described in the words of one who has several times witnessed the effects of an incompatible blood: “The clinical picture of these reactions is typical. They occur early, after the introduction of 50 cc. or 100 cc. of blood; the patient first complains of tingling pains shooting over the body, a fullness in the head, an oppressive feeling about the precordium, and, later, excruciating pain localized in the lumbar region. Slowly but perceptibly the face becomes suffused a dark red to a cyanotic hue; respirations become somewhat laboured, and the pulse rate, at first slow, sometimes suddenly drops as many as from twenty to thirty beats a minute. The patient may lose consciousness for a few minutes. In one-half of our cases an urticarial eruption, generalized over the body, or limited to the face, appeared with these symptoms. Later the pulse may become very rapid and thready; the skin becomes cold and clammy, and the patient’s condition is indeed grave. In