The need for active immunization against Clostridium tetani infection is evident, especially in those instances in which it has been necessary for industrial and constructional workers to receive numerous prophylactic injections of tetanus antitoxin over a comparatively short period of time owing to various individual injuries. This has resulted in development of a hypersensitivity to horse serum proteins with the resulting severe serum reactions after injection which incapacitate the workers for variable periods of time. Active immunization could be adopted advantageously among military forces and in groups that are unusually subject to injury under conditions where C. tetani might exist and the potential danger from tetanus is high. Historical. In 1924 Ramon (1) detoxified soluble products of Cornebacterium diphtheriae using 0.4 per cent formaldehyde. This work was followed by Descombey's (2) observation that the same technic was applicable to the soluble products of C. tetani with subsequent production of tetanus toxoid.