A chymopapain derivative having the enzymatic activity reduced by at least 95% while retaining the original allergenic activity of at least 90% is suitable for skin testing to detect allergic hypersensitivity to chymopapain and for treating patients exhibiting allergic hypersensitivity to chymopapain. The enzymatic activity can be blocked by reacting thiol groups of the enzyme with and iodoacetic acid, bromoacetic acid, or a salt, ester or amide derivative thereof, under conditions which block the undesirable enzymatic activity while retaining the desired allergenic activity.