Because immediate hypersensitivity reactions can occur in individuals exposed to powdered pancreatic extracts, 36 patients with cystic fibrosis and 51 patents of such patients wwer studied for evidence of sensitization. Sensitivity to the extracts as evidence by history and skin testing was infrequent in the children with cystic fibrosis. However, skin testing for immediate hypersensitivity with either crude pancreatic extracts or inactivated trypsin correlated well in their patents with a history of clinical symptoms. IgE mediation of these reactions in sensitized individuals was demonstrated by antigen-induced histamine release from leukocytes, passive transfer studies, and immediate response to inhalation challenge.