Abstract An explanation of experimental allergic encephalomyelitis prevention and suppression is presented based upon evidence that the active unit in disease induction is an encephalitogen- adjuvant complex. The stereochemical complementarity in structure of the encephalitogen and adjuvant is mirrored in complementarity in the recognition sites of lymphocyte populations activated against encephalitogen and adjuvant. Since two complementary lymphocyte populations are necessary for disease induction, any procedure that prevents the development of one of these populations will prevent disease induction. Any procedure that eliminates one population after induction has occurred will suppress the disease. We argue that all extant data support the hypothesis. Several new experiments are proposed to further test it.