Two dissimilar proteins, designated as components I and II, of botulinum C2 toxin elaborated by strain 92-13 were purified to a homogeneous state. The molecular weights determined by sodium dodecyl sulfate gel electrophoresis were 55,000 for component I and 105,000 for component II. Whereas each component showed no or feeble toxicity even after being treated with trypsin, the toxicity was elicited when these two components were mixed and trypsinized. The toxicity of the mixture of components I and II at a ratio of 1:2.5 on a protein basis was 2.2 X 10(4) mouse intraperitoneal 50% lethal doses per mg of protein and increased by 2,000 times or more when treated with trypsin. These results indicate that the molecular characteristics of botulinum C2 toxin differ from those of the toxin of Clostridium botulinum types A through F in that C2 toxin is constructed with two separate protein components, which are not covalently held together, and that its toxicity is elicited by cooperation of the two components.