The development by Enders et al.1 of a live attenuated measles-virus vaccine, the evolution of a regimen for modifying the clinical reactions to the vaccine,2-4 and the proof of efficacy of the vaccine so administered have provided the scientific background and rationale for widespread application of the vaccine in the human population. This important development created the need, also, for precise measures of control of the safety of the vaccine which have not been defined to the present time. Most important, in this regard, is the need for reliable procedures for differentiating between and for identification of virulent and attenuated measles-virus strains. Such requirement is common to all live virus vaccines and was emphasized most clearly in the development of the in vivo and in vitro genetic marker tests for control of the live poliovirus vaccine.5
Studies in our laboratories have revealed reproducibly demonstrable differences between representative