A modified purification method was used to isolate the neurotoxin of proteolytic Clostridium botulinum type B strain Lamanna. The preparation was found to be a mixture of two protein forms. They were of molecular weight 152,000 and could not be separated by ion-exchange chromatography or electrophoresis in polyacrylamide gel. One was a single polypeptide chain, and the other was a dichain molecule (nicked toxin) held together by an interchain disulfide bond(s). Trypsinization increased the toxicity of the toxin preparation and converted the single-chain molecules into dichain forms that were indistinguishable from the endogenously generated nicked toxin. A protease of the type B culture, with substrate specificity similar to that of trypsin, did not change detectably the molecular form of unnicked type E toxin, although toxicity was increased. Higher toxicity was obtained when unnicked type E was trypsinized; the resulting preparation contained only nicked toxin molecules.