Repeated administration of L-asparaginase leads to the development of specific antibodies and hypersensitivity reactions. The aim of the study was to evaluate a possible cross-reaction of anti-asparaginase antibodies, developed against the native E. coli L-asparaginase (Asparaginase Medac), with other preparations of the enzyme. Sixteen patients with acute lymphoblastic leukemia, in whom in the reinduction phase of treatment hypersensitivity against L-asparaginase was observed and/or the presence of anti-asparaginase antibodies was established were recruited for the present study. Ten out of 16 tested sera showed cross-immunoreactivity to PEG-asparaginase, while no reactivity to L-asparaginase derived from Erwinia chrysantemi was observed. Since cross-reacting antibodies were also found in sera of patients with no overt allergic reaction, L: -asparaginase may undergo silent inactivation during the reinduction phase of therapy. This finding is of clinical importance with regard to appropriate dosage and necessitates careful enzyme activity monitoring in all patients undergoing repeated treatment with various L-asparaginase preparations.