After a brief survey of the history of Diphtheria, culminating in the discovery of antitoxin, an attempt has been made to review the changing customs as regards dosage methods of administration of the latter, and the trend of modern opinion has been indicated. A series of 130 cases of toxic Dphtheria has been treated, and a method described, by which large doses of antitoxin, diluted if necessary, can be given intravenously. Consideration has been given to the questions of the optimum dilution, and what is the best diluent of antitoxin. An analysis has been made of the complications of Diphtheria and of the serum phenomena occurring in the series. Observations have been recorded on Vitamin C. metabolism, in Diphtheria, and on adjuvant therapy with Vitamin B1., Cortin, Glucose, and streptococcal antitoxin.