When the Stockholm and 468C strains of type C and the 1873 strain of type D Clostridium botulinum are "cured" of their prophages, they simultaneously discontinue the production of their dominant toxins (C(1) and D), but they continue to produce a second antigenically monospecific toxin (C(2)). These "cured" strains of types C and D therefore become indistinguishable with respect to the toxin produced. Fifteen type C cultures received from other laboratories discontinued to produce the dominant toxin when subcultured in broth. The C(2) toxin, however, was produced by eight of these cultures. The C(2) toxin is produced by these cultures as a protoxin that requires treatment with trypsin before its toxicity can be demonstrated. Of the 21 type C cultures that produce the C(1) toxin, 20 were shown to produce the C(2) toxin. The filtrates of 14 of these cultures required trypsin treatment before the C(2) toxicity could be demonstrated. Low levels of toxicity could be demonstrated in the six remaining culture fluids without trypsin; toxicity, however, was increased with trypsin.