During recent years evidence has gradually been accumulating to suggest that the best and most economical method of immunizing children against diphtheria is by two suitably spaced doses of alum-precipitated toxoid (A.P.T.). Experience, however, of this method has so far been very limited. The great immunizing campaigns in the United States, Canada, and France have been carried through mainly with toxin-antitoxin mixture and with formol toxoid. Most of the early trials of A.P.T. were made by the onedose method, which is now generally recognized to be un5atisfactory. The number of careful observations made