Abstract The relative lack of sensitivity of intracerebral virus-neutralization tests in mice, and their consequent limitations when applied to the investigation of certain epidemiologic or immunologic problems, is generally recognized. Inasmuch as the ability to detect or quantitate neutralizing antibodies in a serum is largely determined by the route by which the serum-virus mixtures are injected into the animal,3 it would appear that a more sensitive test might be achieved by inoculating the mixtures by a route other than the cerebral. In order to do so, however, mice of known age must be employed, as susceptibility to peripheral inoculation, unlike that to cerebral inoculation, usually diminishes markedly with increasing age.