Four hypotheses have been related to the production of toxoid by the action of formaldehyde upon diphtheric toxin: (a) the toxin contains an amino-group or groups with which formaldehyde reacts; (b) the reaction is a (partial) hydrolysis in which formaldehyde acts as a catalyst; (c) combined and not free formaldehyde converts toxin into toxoid; (d) formaldehyde causes a reaction of condensation in which 2 or more molecules of toxin unite to form an atoxic compound. Barikin, Kulikow, Minerwin, and Kljuchin (2) used several aldehydes, and stated that the smaller the molecular weight of the aldehyde the more quickly the toxicity disappeared and the firmer the union of aldehyde and amino-groups became. Kissin and Bronstein (15) found that only a part of the amino-nitrogen in toxin disappeared during the toxoiding process, and concluded that only the NH2-groups responsible for toxicity reacted with formaldehyde.