1) The present research has been planned to clarify the importance of tryptophan residues in chymopapain A by means of N-Bromosuccinimide (NBS) oxidation. 2) About four tryptophan residues were oxidized and the first oxidizable tryptophan residue was important for enzyme activity. 3) The rate of oxidation by NBS increased with the decrease of pH. 4) SH group and histidine residues in chymopapain A were not affected by NBS. 5) These results indicate that the first oxidizable tryptophan residue is responsible for the loss of enzyme activity by NBS oxidation.