C2 toxin (C2T) elaborated by Clostridium botulinum types C and D is composed of two dissimilar protein components, designated components I and II. The biological activity of C2T is enhanced by treating the toxin with trypsin. This activation of C2T is observed as a result of mixing untrypsinized component I and trypsinized component II but not as a result of mixing trypsinized component I and untrypsinized component II. The data presented here show that the maximum lethality of C2T, determined by mixing untrypsinized component I and trypsinized component II, was attained by treating component II with trypsin at a ratio of 10:1 on a protein basis for 30 min at 35 degrees C at pH 7.5. The activation of component II was always accompanied by a change in the molecular weight of the component from 101,000 to 88,000 as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). However, the gel filtration of trypsinized component II resulted in the separation of two active components, with apparent molecular weights, estimated from the elution volume by gel filtration, of 365,000 and 74,000. The high-molecular-weight component II had hemagglutination and hemolytic activities, whereas the low-molecular-weight component II has only hemagglutination activity. These two molecular species of active component II had approximately the same lethality, when mixed with component I, and gave a single band in SDS-PAGE, with a molecular weight of 88,000, the same as that of trypsin-activated component II under different reaction conditions. The results indicate that the activation of C2T by trypsin is due to the molecular conversion of component II from molecular weight 101,000 to 88,000 as determined by SDS-PAGE and that the trypsin-activated component II tends to form an oligomer of the active component II.