Previously reported experiments (1, 2, 3) on anaphylaxis apparently due to the toxin or toxoid antigen were made upon actively sensitized (antitoxic immune) animals. The results had indicated that while other diphtheria antigens participated to some extent, the major part of the anaphylaxis could be assigned to the toxin or toxoid antigen, provided one used both for the immunization and for the intravenous test minimal amounts of unheated filtrate in which toxin or toxoid was the dominant antigen, and selected for intravenous test the individual guinea pigs that had been “earliest” in antitoxin production. Passive sensitization possesses certain advantages over active sensitization in the distinction of reactions due to toxin-antitoxin from reactions due to other diphtheria antigen-antibody combinations, for with the passive method it is possible to examine each of the sensitizing sera for antibodies other than antitoxin and, when desired, to remove the other antibodies by absorption of the antisera before using them for sensitization.