In recent studies Webster (1) has shown that only a limited number of the anti-rabic vaccines made in this country are capable of immunizing mice against rabies. Since it is comparatively easy to protect these animals with a sufficiently potent vaccine we must infer from Webster's work that much of the available vaccine is of a doubtful usefulness. The problem of providing more satisfactory vaccines is a complicated one involving both scientific and manufacturing questions; as such, a satisfactory solution will be reached slowly. Nevertheless it is feasible now to introduce practical tests that would eliminate the demontrably weak and worthless vaccines. The complete assay of existing products that this implies falls under two heads: (1) a study of the ability of vaccines to immunize against the virus from which they are made and (2) an investigation of the protective values of vaccines which are potent against such homologous virus when tested against other strains of rabies virus.