The isolation of a new type of mutant Corynephage beta, which carries a missense mutation in the structural gene for diphtheria toxin synthesis is described. The lysogenic C7(8)(beta(197))(tox-crm+) strain of Corynebacterium diphtheriae produces a nontoxic, extracellular protein of molecular weight 62,000. This protein is immunologically indistinguishable from toxin itself but inhibits the action of toxin on HeLa cells, probably by competing for attachment sites on the cell membrane. In contrast to fragment A derived from diphtheria toxin, fragment A(197) is unable to catalyze the inactivation of eucaryotic polypeptidyl-transfer RNA-transferase II. When mixtures of the two nontoxic mutant proteins, enzymically active crm(45) protein and enzymically inactive crm(197) protein, are subjected to mild treatment with trypsin in the presence of a thiol and then allowed to reoxidize after dialysis to remove excess thiol, "diphtheria toxin" is reconstituted in high yield.