Trypsin (EC 126.96.36.199) and Chymotrypsin (EC 188.8.131.52) are proteolytic pancreatic enzymes which are secreted as inactive precursors trypsinogen and chymotrypsinogen, respectively. They have several clinical pharmacological as well as laboratory applications and they are used in protein chemistry, especially in the preparation of insulin. Exposure to enzyme dusts has long been known to cause occupational immediate hypersensitivities of the airways. Also trypsin and chymotrypsin are potential inhalable sensitizers; cases of specific airway sensitization caused by trypsin and chymotrypsin containing products could be shown clearly by several studies. Positive skin prick and challenge tests as well as specific IgE antibodies have been described. Since these results and the clinical symptoms usually matched well, an immunological mechanism of action is suggested. There is no clear evidence of allergic cell‐mediated late type eczematous skin reactions. Trypsin and chymotrypsin are designated with “Sa” (for substances causing airway sensitization) but not with “Sh” (for skin‐sensitizing substances).