L-asparaginase therapy is often limited by allergy or toxicity and probably in some cases by antibody mediated inactivation of the enzyme. These problems can be avoided by extracorporeal application of l-asparaginase. As the enzyme is a stable tetramer with high molecular weight it cannot pass through the dialysis membrane in contrast to the amino acid l-asparagin which is destroyed. The resulting l-asparagin depletion of the plasma is sufficient for therapeutic success. Effective extracorporeal l-asparaginase therapy is demonstrated in two patients with ALL who were resistant to other chemotherapy and could not be treated intravenously because of allergy and toxicity.