Immobilized asparaginase was prepared by embedding asparaginase (which is effective for remission in children with leukemia) into fibrin polymer formed by fibrinogen- fibrin conversion in the presence of thrombin. The immobilized asparaginase film did not dissolve in 6 mol/I urea, suggesting that blood coagulation factor XIII participates in the cross-linking between fibrins and between fibrin and asparaginase.