The influence of two different E. coli asparaginase (ASP) preparations on fibrinolytic proteins in childhood ALL was recently reported, demonstrating a clearly significant association between ASP activity and haemostatic changes. Since the Bayer preparation is no longer available for treatment of large series of patients with ALL, the present study was designed to prospectively evaluate coagulation and fibrinolytic changes in leukaemic children receiving different doses of Medac ASP, which is now available for treatment of childhood ALL. Leukaemic children in whom ASP Medac was administered at 3 d intervals in a two-step dose reduction (5000 IU/m2, n = 10; 2500 IU/m2, n = 15) were compared with children who had received Bayer ASP 10,000 IU/m2 in the same time schedule in a former randomized trial; at the same venipuncture, blood samples for coagulation studies were obtained before each ASP administration together with serum samples for pharmacokinetic monitoring. Compared with Bayer ASP 10,000 IU/m2, patients receiving Medac ASP 5000 IU/m2 showed significantly decreased values of fibrinogen, plasminogen, and alpha 2-antiplasmin, along with significantly enhanced thrombin generation. Improvement occurred in children treated with 2500 IU/m2 Medac ASP; alpha 2-antiplasmin and D-dimer no longer differed from the Bayer group. Since both patient groups showed complete asparagine depletion during the course of ASP administration, the lower dosage of 2500 IU/m2 administered at 3 d intervals should guarantee the specific metabolic therapy for ALL, leading to depletion of the circulating pool of asparagine.