When one considers with these facts the frequency of paralysis and of permanent hypermetropia, the loss to the cause of education through school closure or exclusion, let alone the tremendous cost to the State of hospital accommodation for this disease ---when one considers these facts, I say, one finds great incentive for work on this disease which Sir George Newman rightly terms one of the most baffling from the point of view of the Public Health administrator. A well-defined etiology, a comparatively easy bacteriological diagnosis, a specific anti-serum with generally recognised prophylactic and curative powers - - - ail these are ours; yet this disease breaks out and rages amongst the population more than ever; and in this thesis I propose to describe my work in connection with Diphtheria, hoping that some little light may be thrown on this very important subject.