The knowledge of the nature of vaccinia virus gained in recent years and the methods of preparing relatively pure suspensions of the virus which have been developed justify the hope that the technique of vaccination against smallpox may be improved. The potency of vaccine lymph and its relative freedom from bacterial contaminants have been regulated under the Therapeutic Substances Act for several years past; nevertheless, for prophylactic purposes, it is desirable to introduce the use of pure bacteria-free virus in a measured volume of known potency. The usual method of vaccination which involves scratching the skin and applying to the scratch a variable amount of virus-containing lymph cannot be described as an accurate procedure and, unless proper precautions are taken, may invite local secondary bacterial infection. If it could be shown that a satisfactory immunity follows the intracutaneous injection of a measured amount of a bacteria-free suspension of virus, this method would possess obvious advantages; the usual dressing could be dispensed with, the amount of scarring, and the local secondary bacterial infection which is associated with vaccination would be reduced or abolished. In our opinion, the value of vaccinial scars has been exaggerated; they may be useful as visible evidence that the patient has been successfully vaccinated sometime in the past, but they are not an indication of existing immunity to smallpox. It is not usually suggested that any other measure of prophylactic inoculation should be accompanied by a permanent scar as a record of immunization.