As is well known, immunity in diphtheria is generally incomplete and of short duration. Even an attack of this disease with consequent stimulation of the body cells with diphtheria toxin and the formation of homologous antitoxin and other antibodies does not confer for any considerable length of time that immunity which is so characteristic of scarlet fever, measles and small-pox. Passive immunization with antitoxin confers an immunity even more fleeting, because the immune serum constituents

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... e quickly eliminated or destroyed. For this reason, Smith, Park, and more recently von Behring have suggested an active form of immunization in diphtheria by means of toxin-antitoxin mixtures, as the antitoxin prepared by our own body cells is more lasting. In order to estimate the degree of immunity following injections of these toxin-antitoxin (T-A) mixtures, it is necessary to determine the amount of antitoxin per cubic centimeter of serum in units or fractions of a unit. For this purpose R\l=o"\mer'sintracutaneous technic with the guinea-pig has been used, but the method is rather too uncertain and complicated for the routine examination of large num¬ bers of persons. Schick1 has proposed a very simple clinical test for this purpose; it is based on the observations of von Behring that as little as onehundredth of a unit of antitoxin per cubic centimeter of our serum will protect a person against diphtheria. Schick has determined that when an amount of diphtheria toxin equal to one-fiftieth the minimal lethal dose (M. L. D.) for a guinea-pig weighing from 250 to 300 gm. is injected intracutaneously in the human skin, if one-thirtieth or more antitoxin per cubic centimeter of serum is present this dose of toxin is neutralized and no reaction follows; if less than this amount of anti¬ toxin is present or none at all, the toxin injected remains unneutralized and produces a local inflammatory reaction of erythema and edematous