HIS report presents a case of allergic sensitivity to chymotrypsin (bovine) in a laboratory worker. The sensitivity apparently was induced by inhalation of the powdered material. With the increasing use of purified enzyme preparations, the hazard of sensitization should be recognized. A larger number of scientific workers are being exposed to inhalation of the dried materials. Injectible chymotrypsin is being used to reduce tissue swelling following trauma and infection.’ The other pancreatic proteolytic enzyme, trypsin, is used medically, both by injection in inflammatory disorders”, 3 and in the form of an aerosol to liquefy mucus in cases of asthma, bronchitis, and bronchiectasis.4, 5 Chymotrypsin is produced by the action of trypsin on chymotrypsinogen. It acts on the peptide link in the carboxylic side of aromatic amino acids. Because of the proteolytic action, the enzymes have been used to dissolve mucus or fibrin. During tests for specific skin sensitivity, the proteolytic activity of trypsin and chymotrypsin, with its resulting irritative wheal and tla,re, introduces the factor of nonspecific reactions, and this must be evaluated in each individual test. To reduce this source of error, we used the nonproteolytic precursor of chymotrypsin (chymotrypsinogen) in the passive transfer tests reported below. Chymotrypsinogen and chymotrypsin have been reported to be distinguishable by means of the Schultz-Dale reaction in the guinea pig$ however, t,hey are very closely related immunologically. Their cross reactivity is sufficiently strong to have permitted the use of chymotrypsinogen to test for chpmotrypsin sensitivity in the case reported here.