In most cases of severe hæmorrhage the patient has probably not lost more than 1,400 to 1,800 cc. of blood, and 600 to 800 cc. will be enough to restore the balance of the circulation. This is in practice the amount of blood that is commonly administered, and it is well within the limits of what a single blood donor can afford to lose. If a more definite standard be required, it may be laid down that in a single transfusion for acute anæmia 750 cc. of blood should be given. If, in an exceptional case, more than this is needed, a second transfusion should be performed with a similar amount taken from another donor. Sometimes it may happen that a patient already _in extremis_ from loss of blood, needs a severe operation; in such a case a second transfusion may be given with great advantage at the conclusion of the operation. The first transfusion will restore the patient sufficiently to render the performance of an operation possible; the second will combat the additional shock and hæmorrhage which it has caused. It has already been stated that it was uncommon during the war to meet with patients who were suffering from anæmia uncomplicated by traumatic shock. It was in fact the condition of shock which tended to dominate the clinical picture, and it was towards the elucidation of the facts concerning shock, its causation, prevention, and treatment, that the investigations co-ordinated by the Medical Research Committee were mainly directed. These investigations were carried out both in the laboratory and in the military hospitals, and considerable additions were made to the knowledge of the condition. It is necessary to give some account of the conclusions which were reached in order that the rôle of blood transfusion in the treatment of shock may be fully understood. Hæmorrhage and shock cannot be dissociated, and this is not only because they so frequently occur together in the same patient, but also because the manifestations of the two conditions are essentially the same. In shock, as in hæmorrhage, are found the same pallor of the face and mucous membranes, the same fall of blood pressure and rapid pulse, the same perspiration, restlessness, and shallow respiration. The symptoms following a severe hæmorrhage have sometimes been referred to as constituting a “shock-like condition.” As will be seen, however, it is more accurate to describe the symptoms of shock as closely resembling those of hæmorrhage, and to regard both conditions as a manifestation of deficient fluid content in the circulation.