In the United States vaccination against smallpox usually is accomplished by percutaneous inoculation of calf lymph employing the multiple pressure technique. This method is time consuming; moreover, success or failure in eliciting a vaccinial reaction depends largely on the skill of the vaccinator. Because of these drawbacks another method was sought which might be more suitable for the rapid immunization of large groups of people by relatively untrained personnel. The use of a jet injection apparatus for inoculating vaccinia virus appeared to be worth investigating for a number of reasons. A known quantity of material is introduced into the recipient, a portion of which lodges intradermally while the majority reaches the subcutaneous tissues forming a cone shaped pattern with the apex at the point of entrance (1). Since no needle is used, contamination of the instrument by such blood-borne microbial agents as malaria or serum hepatitis is avoided and frequent sterilization of the jet injection apparatus is unnecessary.