It is generally accepted that the prophylactic immunization of human beings with living or dead preparations of the typhoid bacillus protects to a marked degree against typhoid fever. This protection is particularly demonstrable in segregated bodies like armies where immunized and non-immunized men are subjected to even chances of infection. In spite, however, of the convincing evidence of this protection by vaccination in the aggregate, we have as yet little evidence on the actual duration of any method of immunization, and no certainty as to security of an immunized individual. Firth,1 to be sure, on the basis of the scientifically assembled statistics from the British Army in India, has ventured the statement that anti-typhoid immunizations may be expected to protect absolutely for two and a half years, with further evidence that partial immunity is enjoyed for four or five years. It would aid enormously in estimating the