Carboxypeptidase G,, an enzyme capable of cleaving the glutamate moiety from a variety of folate analogs, has been immobilized on nylon tubes and hollow fibers for use in extracorporeal enzyme reactors for methotrexate (MIX) removal from blood. The stability and reactor parameters of the system have been investigated with the use of single tubes and a multitubular arrangement. The results are used to predict their MTX-removing capacity at flow rates and MIX concentrations of clinical interest. In vivo experiments with dogs demonstrate the potential appli cations of such extracorporeal shunts in "rescue" after administration of large doses of MIX. For dogs a Carboxy peptidase G, reactor with a clearance value of about 150 ml/min would be required in such application.