This paper reports the anaphylactic reactions of guinea pigs immunized with toxoid and subsequently injected intravenously either with phenolized filtrate containing the toxin antigen or with formolized filtrate containing the toxoid antigen. It was proved that the acute death of the antitoxic immune animals represented true anaphylactic reactions specific to bacterial products contained in diphtheria filtrate. While it was recognized that other constituents participate to some extent, the evidence as a whole indicated that the toxin or toxoid antigen is the principal agent of the anaphylactic reaction of antitoxic immune guinea pigs to unheated filtrates containing a relatively large proportion of toxin or toxoid. The complications due to the substances other than toxin or toxoid are least prominent if one uses both for the immunization and for the intravenous test, minimal amounts of unheated filtrate in which toxoid is the dominant antigen, and finally selects for intravenous test the individual guinea pigs that had been “earliest” in antitoxin production. As might be expected, the complications become more prominent if either the immunization or the intravenous anaphylaxis test is made with diphtheria filtrate in which toxin or toxoid is not the dominant antigen (e.g., filtrates of weakly toxicogenic strains or heated filtrates of toxicogenic strains). Most of the experiments were made with toxoid-immunized guinea pigs, but after a proper rest period animals immunized with toxin-antitoxin gave anaphylactic reactions to intravenous injection of formolized toxoid-containing filtrates.