For a physician the choice among these three forms is governed by considerations of efficiency, safety, and ease of use. All the forms are under certain conditions efficient, but comparative tests show that the emulsion of the pulp issued by different laboratories is much more certainly efficient than the other forms, and the glycerinated emulsion is at present in most general use both abroad and in this country. It is also true that all forms may be perfectly safe. All forms contain bacteria when prepared, and the majority of these bacteria die within a few weeks or months after preparation. On account of the mildly antiseptic quality of glycerin the bacteria in the glycerinated emulsion usually die sooner than those in the other forms of virus, and so far as bacteria are objectionable in the virus the glycerinated form may therefore be said to be somewhat preferable. It should be added, however, both that glycerinated virus is usually put in the market before the bacteria have disappeared and that the bacteria present in virus issued by well-conducted laboratories are not found to be pathogenic to persons when inoculated by the customary method of vaccination. The ease of use of any form of virus depends largely upon the custom of the physician. In vaccinating a large number at one time there can be no question that the use of a liquid virus supplied in vials is more rapid than the use of a dried virus, as the latter has to be thoroughly moistened before it can be applied effectively. *Methods of Vaccination.*--The usual method of vaccination is to scarify a spot on the skin and to rub the virus on that spot. The choice of place depends partly on æsthetic reasons and partly on convenience. To avoid the formation of an unsightly scar on the arm, the leg may be used instead. If the arm is chosen, the insertion of the deltoid is the place of election on account of the small number of lymphatics there. If the leg is chosen, the area just below the head of the fibula presents the same anatomical advantage; but a spot a short distance above the knee on the outside of the thigh is often thought to offer less opportunity for injury and infection. Choice between the sides depends in an adult on the use to which the vaccinated limb is to be put, and in a baby on the advantage of vaccinating the side which is carried away from the nurse.