The tryptophan content of Clostridium perfringens epsilon toxin was investigated. When the tryptophan content was determined by amino acid analysis after the hydrolysis of epsilon prototoxin with methanesulfonic acid containing 3-(2-aminoethyl)indole and by the spectrophotometric method with N-bromosuccinimide, the number of tryptophan residues was calculated at 1/mol of the protein. Cleavage of the prototoxin or the toxin with N-bromosuccinimide in the presence of urea gave two new fragments on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. There was only a tyrosine residue as the new N-terminal amino acid after the cleavage of the prototoxin or the toxin with N-bromosuccinimide. The data showed that epsilon prototoxin or epsilon toxin contained only one tryptophan residue.