ACTIVE immunization against diphtheria has developed during the past 15 years, particularly in North America, into one of the most important specific measures for the prevention of disease. Prior to the use of toxin-antitoxin mixture, smallpox vaccination was considered the most successful individual specific procedure in preventive medicine; but improvements in methods of diphtheria prophylaxis have been so great in the past few years that one can safely say that immunization against diphtheria is at least as satisfactory as vaccination against smallpox. Since no one now-a-days questions the effectiveness of immunization campaigns on incidence of diphtheria as reflected in both cases' and deaths, it is only necessary to cite in passing, the experiences of Auburn, N. Y.,"2 Grand Rapids, Mich,' and New York City,4 without going into detail as to the tremendous reduction in both cases and deaths in these places, for which accurate records are available. We are familiar with the extreme difficulty experienced by workers in reaching the most highly susceptible children for immunization, and since these children are reached with such difficulty it is important that we use the immunizing agent which will produce immunity in the greatest number of subjects injected. Toxin-antitoxin mixture and toxoid are both being used and we all wish to know which, if either, is the superior product, and the advantages and disadvantages of both. Toxin-antitoxin mixture is made from mature toxin by adding sufficient diphtheria antitoxin so to decrease its toxicity that it may be safely injected. This delicate balance is readily attained by experienced manufacturers working with toxins and antitoxins, the values of which are accurately known. In the beginning mixtures were made by adding antitoxin to the undiluted toxin-hence the original mixtures containing from 3 to 5 L + doses of toxin per human dose. Some of us will probably re-