It must not be supposed, however, that the withdrawal of even 1,000 cc. of blood will usually have an appreciable effect upon a healthy man. It is impossible to predict from the donor’s appearance what immediate effect the loss of blood will have upon him. It sometimes happens that the most robust-looking individual becomes faint after losing a few hundred cubic centimetres, whereas another, to all appearances pallid and much less satisfactory, will not evince the slightest discomfort from the loss of 750 cc. or even more. Normally a man should be able, by his physiological mechanisms, to compensate reflexly and at once for the removal of this amount of fluid from his circulation. In any case, the worst effect that is seen in a well-chosen donor is a transient faintness; it is usually wise to keep him on his back for two or three hours after the operation, and he should not, if it can be avoided, return to his work on the same day. During the late war a medical officer of my own acquaintance gave 750 cc. of blood for a severely wounded friend and continued his arduous duties as Surgical Specialist in a Casualty Clearing Station immediately afterwards. In this case, however, the donor was solely responsible for his own welfare; usually this responsibility rests upon another, and greater care must be exercised. The effect, indeed, of a transfusion upon the donor seems to depend more upon psychological than upon physiological factors. A nervous and excitable donor is more likely to suffer than one who approaches the operation without apprehension. This is another point in favour of employing a professional donor, who soon becomes familiar with the whole procedure and will lose all symptoms of fear. The same considerations may be applied to the use of women as blood donors. In them the spirit of self-sacrifice is commonly more highly developed than it is in men, and some of the most eager donors will be found among them. The disability of nervousness will, however, occur more often in women, and another consideration of importance is that the veins of a woman are usually much less easily accessible than those of a man. Not only is the abundant subcutaneous fat an impediment in women, but usually the superficial veins are all of small size. The method of choice for performing a blood transfusion will be presently described, and it will then be seen that the operation is easier and that much less damage is inflicted on the donor if a large superficial vein can be tapped. In women this will very often be difficult or even impossible. In general, therefore, it may be stated that the use of women as blood donors is to be avoided. The fallacies concerning the indiscriminate transfusion of an infant with its mother’s blood and of any patient with the blood of a near relation have already been explained. TESTING FOR BLOOD GROUPS