Studies were carried out on sera from eight subjects hyperimmunized with toxoid who developed marked immediate skin reactivity to toxoid associated with circulating non-precipitating antitoxin. With the use of the rabbit skin test, the agar diffusion technique, and three different methods for passive transfer of skin sensitivity, it was possible to obtain detailed qualitative and quantitative data relating to the antitoxin in these sera. It was found that specimens from six individuals contained only skin-sensitizing antitoxin. Two sera showed a lack of parallelism between antitoxin titers as obtained by rabbit skin test and titers as demonstrated by tests in human skin. It was presumed that these sera contained two different varieties of non-precipitating antitoxins, and that only one of them was skin-sensitizing antitoxin.

A new technique is described for measurement of skin-sensitizing antitoxin. Its specificity is based upon the ability of this antitoxin to remain at skin sites and later neutralize the delayed specific toxic effects of intradermal Schick test reagent.

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