Occasionally during the last two years advertisements for blood donors have appeared in newspapers, probably not in vain. If the demand for blood donors becomes greater than it has been as yet, it will certainly result in the creation of a class of “professional blood donors,” who already exist in some numbers in the United States of America, where blood transfusion is a more widely recognized form of therapeutics than it is in this country. These professionals have even formed a Trade Union, so that as high a fee as possible may be obtained from those who need their blood. Apart from this, some of the advantages of having these professionals available have already been explained in the chapter on the dangers of blood transfusion. It is evident that certain sources of danger can be eliminated in advance, and in an emergency it is obviously better to have donors of known groups available, so that no time is lost in testing the prospective donors of whom several in succession may be found unsuitable. Probably it will be easier for practitioners to arrange for such professionals to be available at the shortest notice than for necessary arrangements to be made in a hospital. Even in large institutions it is usually difficult for any of the men employed in them to be spared from their work for twenty-four hours, so that, although suitable men of known groups are always within call, it may be impossible to use them. This, however, is not the place to discuss the organization that is necessary to make a blood transfusion a really efficient form of emergency treatment in a hospital. It may merely be observed that in every hospital it should be possible to give a blood transfusion to a patient suffering from urgent hæmorrhage within fifteen minutes of his arrival on the premises. Whether the donor be a “professional” or an “amateur,” it may be useful to mention a few points to be observed in choosing him. There can be no doubt that the most satisfactory individuals for the purpose are young men between the ages of eighteen and twenty-five. The younger the donor, the less likely is he to be suffering from certain of the diseases mentioned in the chapter on the dangers, the less will be the immediate effect of the withdrawal of circulating fluid, and the more quickly will he recuperate from the loss of blood.