L-asparaginase, an enzyme with established antileukemic activity, increases the induction rate and duration of remission of acute lymphoblastic leukemia when added to vincristine and prednisone for induction therapy. Enzymes derived from two different bacterial sources (Escherichia coli and Erwinia carotovora) are in common use. These enzymes may be associated with toxic reactions of differing frequency and severity. Specifically, the complication of enzyme-induced hyperglycemia may be seen more frequently after exposure to the E. coli product. The authors present two patients in whom it was necessary to substitute the Erwinia enzyme for the E. coli enzyme because of the occurrence of severe allergic reactions to the E. coli enzyme. Hyperglycemia induced by the first product improved after the substitution, suggesting that the Erwinia enzyme may be less diabetogenic than the E. coli enzyme.