Strains of Corynebacterium diphtheriae infected with tox* corynebacteriophages synthesize a powerful exotoxin. Nontoxinogenic, nonlysogenic C. diphtheriae upon infection with certain phage mutants produce altered nontoxic proteins which cross react with antitoxin (CRM mutants). This phenomenon has provided evidence that diphtheria toxin is specified by phage DNA. The role of toxin in the life cycle of corynebacteriophages remains to be elucidated. To investigate factors controlling toxin synthesis at the molecular level, especially the regulatory role of iron, it was desirable to develop a cell free protein synthesizing system in which de Novo synthesis of toxin could be detected and its control studied.