N A SERIES of papers, the first of which appeared in 1923, Ramon has emphasized the fact that diphtheria toxin may be rendered atoxic by the action of formalint and heat; and further that this non-toxic product retains in a high degree its antigenic properties. This new diphtheria antigen has been called anatoxine by Ramon. In England and America the word toxoid is used as synonymous with the word anatoxine. Diphtheria toxoid appears to offer certain advantages as an immunizing agent against diphtheria, over toxin-antitoxin which has been used heretofore. First, as tested on guinea pigs, diphtheria toxoid is wholly non-toxic, whereas toxin-antitoxin is somewhat toxic; and secondly, since toxoid contains no serum its use as an antigen cannot bring about serum sensitiveness. However, the relative merits of the two products have not yet been finally determined. The method for the preparation of diphtheria toxoid as carried out at the Connaught Laboratories is a modification of the method recommended by Ramon. First, it is essential to obtain a good toxin. This is done in the ordinary way by cultivating the diphtheria bacillus in veal infusion broth. About the seventh day after seeding, the broth culture is clarified by filtration through paper pulp and